- Use a power attack on them when they are about to die in melee with Vampire Lord.
 - Talk to them while they are sleeping or try to pickpocket them when they are sleeping to feed.
 - Use vampire seduction on them while and then try to talk to them in the Dawnguard expansion.

Those are all the methods of feeding.
Also if you are a blood-starved Vampire *Lord* you dont have to feed.
These methods work on PC, but I'm not sure about Xbox.