I've already maxed out the vamp lord perk tree, but I never figured out how to do the bite attack. Could I get some help with this?

Good question.

Never seen a power attack bite, just when you thrall someone and feed on them.

Heavy Melee attack as a VL to do a Bite kill move

You have to be in melee form as a VL and you jut do a normal power attack when someone's health is low. You should grab them and bite them.

If you mean how to bite someone who is awake when your a normal vampire use vampires seduction on them and then interact with them.

You have to be in Melee Mode, and you're opponent has to be low on health. Use a Power Attack, and it should bite him.