Instructions
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.