- 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.
