Oblivion, vampires are people infected with an unusual disease, porphyric hemophilia. The player can become a vampire, by choice or accidentally, and can subsequently get cured. In addition, NPCs will be encountered who are vampires, both friendly and enemies.

Effects of Vampirism
Unlike previous Elder Scrolls games, you can pass as a living mortal; citizens and the Imperial Guard will not attack you on sight if you are discovered as a vampire. Yet, also unlike previous Elder Scrolls games, there are only two vampire-related quests and there are no vampire clans that you can join, nor can you associate with or avoid being attacked by vampire antagonists.

Advantages
Regardless of your level of vampirism, you acquire 100% disease resistance and 100% resistance to paralysis. The other advantages and liabilities of vampirism vary depending on how long it has been since the last time you fed on another humanoid.

How quick you become a vampire, feeding on someone will put you back on 25%
0-24 hours (first day) 25%
24-48 hours (second day) 50%
48-72 hours (third day) 75%
72+ hours (fourth day onwards) 100%

Disadvantages
Sun Damage is the greatest disadvantage of being a vampire. At 50% vampirism and higher, you will continually take damage whenever you are outside during the day (6:00 am to 8:00 pm).
The amount of damage increases at higher levels of vampirism.
To prevent sun damage, you need to feed and return to 25% vampirism; as long as you do not sleep after feeding you will remain at 25% vampirism and be able to travel safely.
The level of damage depends upon how bright it is outside
Damage is greatest at midday and less near dusk and dawn.
Cloud cover reduces the damage you take from sunlight, but does not eliminate it. Being underwater has a similar effect.
Walking inside a tent (without needing to enter a door, so you are still "outside"), or standing under something that provides shade has no effect on sun damage.
There is no sun in any of Mehrunes Dagon's Planes of Oblivion, therefore vampires do not suffer from any Sun Damage while in Oblivion. Vampires will still take damage in the Shivering Isles, however.
Whenever you are taking sun damage (even if reduced by cloud cover), you are unable to Fast Travel.
Fast Travel during the day is only possible if it is initiated during the night time. You will not receive damage from travel that is done during the day when using fast travel. However, if you arrive somewhere during the day, you will begin taking damage.
You can fast travel during the day, from any of the houses that you own, except in Anvil.
You cannot wait outside during the day. At night you can use the wait function, but you must stop waiting before sunrise if you are outside. If your character continues waiting while exposed to sunlight, your character will take damage and may eventually die.
Your appearance will change significantly, and will remain so until you are cured. Your character will age by twenty years, acquiring a wrinkled and pale complexion. If your character has an unusual skin color, this may be changed radically. Others will notice and comment on your appearance if you allow yourself to go beyond 25% Vampirism. For humanoids, your eyes will become a pale red color. You may also lose your eyebrows and facial hair. While you are a full vampire (haven't fed in 72 hours or more and have waited/slept three separate times), people may refuse to speak to you, citing your obvious Vampirism. This can be countered by raising the NPC's Disposition to at least 100, for example using the Vampiric Seduction greater power (Charm).
Guards will occasionally attack you if you are at 100% vampirism.

Stages of Vampirism

Becoming a Vampire
There are two ways to become a vampire in Oblivion: by choice, and unintentionally. You can only become a vampire once: if you become a full vampire, and are then cured, you cannot become a vampire again.


By Choice
Complete all the contracts given to you by senior Dark Brotherhood member Vicente Valtieri, and he will offer you the gift of vampirism (available through the quest Darkness Eternal).
Completing the Vampire Cure quest may result in the Count of Skingrad offering to make you a vampire. However, the only way this could conceivably be done is to contract vampirism, start the cure quest, use the Deepscorn Hollow's font of renewal to cure your vampirism, then complete the quest. This may not work even then as Porphyric Hemophilia only turns you into a vampire once, and this is presumably what the count gives you.
Download the Mehrunes' Razor official plug-in and complete the quest. If you eat the Beating Heart ingredient taken from Msirae Faythung (Mehrunes' champion), you will be infected with the diseases Cannibal's Prion and Porphyric Hemophilia. Be sure to let the vampirism take hold first before you use Cure Disease to get rid of the Cannibal's Prion. You can also use the Beating Heart to make a Potion of Porphyric Hemophilia, if your Alchemy skill is 100.

Unintentionally
Whenever fighting vampires you run the risk of being infected with the vampirism disease, Porphyric Hemophilia. Locations where vampires may be encountered are listed at Vampire Dungeons.

Resist Disease effects, even if at 100% or higher, will not decrease the chance of your character catching Porphyric Hemophilia, because of a bug in the way the disease is specified in the Construction Set.[verification needed — what evidence is there that this is a bug instead of an intentional feature?]
Once initially contracted, Porphyric Hemophilia will not take effect until three days have passed, at which point sleeping will trigger the full change into a vampire.

After your first rest/night of sleep (72 or more hours after contracting the disease) you will have strange dreams/visions. You will obtain some vampire skills.
At least a day after your first rest, a second rest/night of sleep will cause more strange dreams/visions, and you obtain some more vampire skills.
At least a day after your second rest, a third rest/night of sleep will cause even more strange dreams/visions, you obtain some more vampire skills, and you become a full vampire and highly vulnerable to daylight.
(Note that at least for the first night of sleep, if you are also due to level up, sleeping will only trigger vampire skills. A second nap will be necessary to increase your level and gain new attributes.)


Preventing Vampirism
For the first 72 hours after being infected with Porphyric Hemophilia, the condition is subject to the rules which govern any other disease (and can be cured in the same fashion). If you become infected and do not wish to become a vampire, you must drink a potion, eat an ingredient (e.g. Mandrake Root), or cast a spell which cures disease. Alternatively, you could go to the nearest temple and receive a blessing at the chapel altar. If you do not cure the disease within three days, however, you can no longer be cured. At that point, the only way to prevent becoming a vampire is to avoid sleeping for as long as possible.


Feeding

In order to feed, a sleeping humanoid must be found. Feeding on an awake, stunned, paralyzed or dead person is not possible. To feed, simply "activate" the person. Doing so while not sneaking will give a choice between Feeding and Talking; doing so while sneaking gives a choice between Feeding and Pickpocketing.

If someone else witnesses a feeding, it will be reported as a crime with a bounty of 40 gold (it is treated as an assault charge). Being caught by someone else feeding also wakes up the victim. Feeding does not kill or harm the person (they are not turned into vampires). Being stealthy while feeding will rarely result in the humanoid waking up, with the exception of Imperial Guards. Guards seem to often be woken up to the feeding and attempt to arrest you as a result. Guards can be avoided as they offer no special feeding bonuses, feeding on any humanoid will garner the same results.

Do note, however, feeding on other vampires is impossible. Attempting to feed on another vampire will not get a feeding dialog box, but instead the vampire will simply react as though feeding was never attempted. They will wake up for conversation if sneak has not being used, or be pick pocketed if sneak is used.

Some easy ways to feed are:

Prey upon one of the beggars that go to sleep around the towns at night. They often sleep in hidden places and guards don't usually go near them. Also, you don't need to break into anywhere.
Feed on your fellow guild members, especially in the Mages Guild which exists in every city. The Fighters Guild works less well, because the porters follow you around. Dark Brotherhood members do not care if you feed on other members. Cloud Ruler Temple also works.
Inns are another source of sleeping people.
Break into a house and feed on the owner. This has some risks: some people do not spend the whole night in their home; the owner might be awake and report you to the guards for trespassing.
Churches, although seemingly unlikely, are great places to feed. Every city has them, and the church members routinely go to sleep at the same time in every church.
If you own the Rosethorn Hall and have the Servants Quarters, you can feed on Eyja (your maid).
If you have the Vile Lair official plug-in installed, feed on the sleeping prisoner in Deepscorn Hollow.
The Gray Cowl of Nocturnal, obtained after completing the Thieves Guild quest line, can be worn when feeding; the assault is then attributed to the Gray Fox rather than the player character.

Advancing Through Stages Of Vampirism
While you are a Vampire, you are always at least at 25% Vampirism. To see this you can open your Journal, select the Spells/Abilities section and then select the far right, Active Effects tab. There should be an effect labeled Vampirism and its number should be at a minimum of 25.

You advance through the stages of Vampirism during sleep or waiting. After 24 hours without feeding, the next time that you sleep/wait, you will advance another 25% in Vampirism. You need to sleep or wait (with the wait function) to trigger the advancement; the advancement will not happen otherwise. Note however, that you do not have to pass time in order to TRIGGER the advancement. In other words, if >=24 hours have passed since your last feeding, you can just start the wait and cancel it immediately.

It seems that the effect of spending long periods of time without feeding, waiting or sleeping has its downside, where even if you feed, the next time you sleep or wait you will still advance in Vampirism (as if your advancements in vampirism were queued-up). This game of catch-up does not seem to last very long, however. In theory, if you were to stay awake a month or just a week and not feed, the catch-up time would be the same either way. In any case, you can simply counter this problem by repeatedly feeding on the same person several times consecutively until you're clear again.

As you progress through the stages of Vampirism, you become more and more light-sensitive. You will notice that light glares more and more, especially when there is a great deal of light around you. At night or in the dark, this can actually help you to see things better if you have a video card with the supported shaders. But during the day this can be impacting, making it difficult to make out the details of anything at all. (Except underwater, where you can see quite well!)

Note: After becoming 100% vampire (not drinking blood for 72+ hours and then sleeping), you will revert back to 25% vampire when you finally do drink blood. Also note; you will lose the abilities of a full vampire and retain the abilities of one being 25%. You will lose a considerable strength bonus, so exercise caution if you are close to your maximum encumbrance limit. This cycle can be repeated as often as necessary.

Curing Vampirism
It is possible to cure vampirism, but it involves a long quest. Once cured, it is not normally possible to become a vampire again (except via console commands). When completing the vampire cure quest, Melisande says that her services will be available if needed in the future.

You still can contract "Porphyric Hemophilia" normally after that, but it behaves just like any normal disease then - drain fatigue 5 until cured, without any special effects from sleeping.

The official plug-in Deepscorn Hollow has a Font of Renewal that can cure vampirism much more easily than going through the Vampire Cure quest. Just like with the standard vampire cure, you cannot become a vampire again after using the Font to cure your vampirism.


Becoming a Vampire Again (After Being Cured)
Although it is still possible to contract porphyric hemophilia after being cured, a character can never become a vampire again without the use of a mod. There are several mods that may work, and there are two ways of doing it via console.

Console
If you want to become a vampire after taking the cure vampirism potion it is possible using console commands. This technique only works once.
Type coc testvampireinterior to be transported to the developer's vampirism testing area.
There are two similar tombstones in the room. First click the one that says "Click me to become a vampire!" then click the second "full vampire" stone.
Feed on the sleeping man and head out.
Return to Tamriel. One way to return is to go out to the main testing hall and use the door labeled "Hawkhaven" to get outside; standard fast-travel is then possible.
Console Method Two
Type in the following commands
Set PCVampire to 2

Player.SetAV Vampirism 25


This will automatically turn you into a 50% vampire, however, you will not be able to advance in vampirism or go back to 25% vampirism, and you will lack the hunter's sight ability as well. Theoretically, you could become a higher up vampire by typing in 50 or 75 instead of 25 in the command, but this does not appear to work. Instead of entering 25, you can enter 1, and your player character will be at 25%.

Vampire Nightmares
As you progress through the various levels of vampirism (25%, 50%, 75%, 100%) you will have nightmares. The first nightmare you experience (changing from non-vampire to 25% vampirism) is always the same. At any other transition you will randomly experience one of ten possible nightmares:

First nightmare: You dream of someone sleeping peacefully in his bed, when a shadowy, gaunt figure silently enters the room. Approaching the bed, the figure leans down and sinks its fangs into the sleeping person. After a few moments, the pale figure rises, blood dripping down his/her chin. As color flows back into the vampire's face, and his/her features fill out, you recognize the face as your own. You awake screaming.
Random Nightmare #1: You dream of long days spent basking in the sunlight of your native lands. You feel the warmth and the heat, and feel your body become refreshed. Then you awaken, knowing it was but a dream.
Random Nightmare #2: In your dream, you see a beautiful young woman holding an infant to her breast. It is only as you draw near that you realize that the woman is a desiccated corpse and the child is purple and bloated, dying of plague. As mother and child crumble to dust, you awaken.
Random Nightmare #3: In your dream, an old wise-woman treats you for burns on your hands. As she applies a salve to your skin, you feel the tingle of magic as the pain begins to subside. But as you watch, the flesh of your hands begins to bubble, crack, and split, falling in chunks to the floor of her hut. As the wise-woman smiles, you wake up.
Random Nightmare #4: In a dream from your childhood you remember playing hiding games with your young friends on a warm summer afternoon. You hide in your parents’ barn, sure you will not be found. Soon, the sweet smell of hay is replaced by a darker, sickly smell. You move deeper into the barn, only to stumble on the rotting corpses of your parents, their throats ripped out. You try to scream as your parents rise and pull you into an embrace.
Random Nightmare #5: You dream of a sumptuous banquet spread out before you. You feast on a particularly choice cut of roasted meat, and its aroma makes your mouth water. It is only as you cut into the last portion that you see the larvae squirming inside. You cough blood as the larvae begin eating their way out of your stomach.
Random Nightmare #6: You lay on your back, with a blissful feeling of peace swirling through your brain. Your every muscle is relaxed. There is no tension in any part of your body. You feel completely at ease. As your vision comes into focus, you notice others standing around you. As a man leans towards you, you recognize the robes of the Necromancer, and you see a glint of light from his scalpel as it begins to cut through your flesh.
Random Nightmare #7: A warm, gentle breeze causes a tickle on your face, but as you go to brush it off, you find you cannot move your arms. Looking at your skin, you realize that it has turned to a brittle, green glass. Standing perfectly still, you breathe in shallow gulps of air, knowing that moving would cause your skin to shatter into thousands of pieces. The tickle on your face worsens, and you know that you are about to sneeze. As your skin shatters, you wake up sweating.
Random Nightmare #8: In your dream, you open your eyes to a beautiful blue sky. The sun is bright, but you feel cool, even a bit damp. You draw a breath but begin to cough, as you expel blood and dirt that you’ve inhaled into your lungs. As you try to draw another, you see a shovel of dirt being emptied onto your face. The sunlight grows fainter as you view it through the soil that covers your body. You would scream, but your mouth fills with dirt before you can make a sound.
Random Nightmare #9: In your dream, you approach a vampire ancient. Having just completed a perilous task for him, you swell with pride, sure that he will now bestow even greater power upon you. The entire clan’s eyes are upon you. Walking towards the dais where he stands, you realize that your task for him is actually unfinished, and that all of your vampiric powers have left you. You cry out as the clan descends upon you, and the ancient’s fangs rip into the flesh of your throat.
Random Nightmare #10: You dream of walking through the cool night air. Your body cries for blood, having not fed for days. Weakly, you stumble to a small pool. As you bend down to it, you see that it is not water in the pool, but warm, fresh blood, steam rising off of it. You lower your head to drink, but cannot open your mouth. As you realize in horror that your lips have been sewn shut, a pair of cold, white hands reach out from the pool and draw you under.

Bugs
When loading a vampire character save from the main start screen, your character's face may be altered to the default face. Female vampires may have their face change into that of an elderly male vampire (although the rest of the body remains female). To avoid this bug, load a different save from the main start screen, and then load your vampire character save from the in-game load menu.
This bug is partially fixed by the Vampire Race Disabler Plugin for OBSE.
This bug is fixed by the Unofficial Oblivion Patch.
Female players can fix their gender by using the Console to enter the command sexchange twice.
The statue in Bruma of your character, which appears after the Great Gate quest has been completed, will always have the default face if your character is a vampire.
There is an obvious bug in the script, monitoring the player's vampire status. The counter for the incubation time is not reset after you cure yourself of Porphyric Hemophilia before becoming a vampire. The result is, next time you catch the disease you will have only 72 hours minus previous incubation time to cure yourself before becoming a vampire.
Advancing the REAL level of a skill past a perk breakpoint (25/50/75/100) while that skill is boosted by vampirism may prevent the perk dialog from being shown, e.g. if you have 25% vamprism (+5 Sneak) and your true Sneak skill is 49 (54 displayed) you will not be told about the perk when you next advance your Sneak skill.
If you equip both a bow and a shield, after feeding, you will be able to see the shield and bow equipped on your character at once (as opposed to seeing the bow and having the shield invisible, and not giving you an armor bonus). There are known cases where you will receive the armor bonus as well as having your bow equipped.

Notes
It is important to remember that until you actually wait or rest, the changes won't be triggered. If you do not want the powers and bonuses of being a full vampire, then it's much better to use spells or potions to heal yourself, and fast travel around the map to consume time, instead of waiting/resting. Although this will in theory allow you to prevent changes indefinitely, you will eventually need to rest in order to level up. In that case, the easiest thing to do is to go to an area frequented by beggars such as the Imperial Waterfront District. Wait in one hour successions until all your changes are complete, then feed on the sleeping beggars. This might be somewhat tedious in the length, though, especially fast traveling to pass time.
If you are planning to become a vampire, one good race to choose is Dunmer, because the 75% Resist Fire will offset the vampire's weakness.
If you are concerned about your character's appearance, feed frequently so that you do not go past the first stage of vampirism. Your character's face will stay relatively normal, e.g., you will look nearly the same as you did before the vampirism. This can also be achieved by loading another save prior to loading the vampire save, (similar to the sexchange bug workaround, above) which results in the character's face being similar to their pre-vampire appearance, without any wrinkles or aging, but retaining the red eyes. After being cured, your face will lose the 'monkey-like' features of the final stages of vampirism, but it will be wrinkled and withered, similar to signs of aging.
Eating Garlic actually does not harm you if you are a vampire. The only vampire affected by the substance is Vicente Valtieri. If he is carrying any garlic - he doesn't have to eat it - he is inflicted with Damage Strength 100pts, Damage Endurance 100pts, Weakness to Normal Weapons 100%, and Weakness to Magic 100%.