In "Elder Scrolls IV: Oblivion," your character can become a vampire either accidentally or by choice through completing certain quests. If you are accidentally infected with "Porphyric Hemophilia," the disease that causes vampirism, and do not wish to remain a vampire, simply drink a cure disease potion within 24 hours. However, if you wish to remain a vampire to gain the bonus attributes, feeding will become a necessary ritual. Feeding, followed by 24-hour wait periods, advances your vampirism by 25 percent at a time until you are a fully realized vampire.
Find a sleeping humanoid. Characters sleep in their designated homes after nightfall, and beggars sleep on bedrolls in the streets. Feeding on animals or humanoids that are awake, stunned, paralyzed or dead will not work. Your victim must be asleep for the feeding option to appear.
Walk up next to your sleeping victim, preferably in sneak mode. This will give you the option to either pickpocket or feed upon the humanoid.
Press the "A" button to activate the feeding option. You will see your vampire character feed on the unsuspecting humanoid.
Sleep or wait for 24 hours and your character will advance to the next stage of vampirism. Do this four times to become a fully realized vampire.