You have to wait until the target is sleeping.
Then, go into sneak mode and click as if you are going to pickpocket them.
You will then be presented with a screen asking you if you want to feed on the target or pickpocket them.
Just make sure that nobody is watching, because you will get into trouble with the law and will have to pay your bounty or go to jail (because feeding on people is illegal).
gaming.stackexchange.com/users/10220/BigStuuu
You have to be in human form then use vampire seduction then try to talk to the seduced.
It will ask you if you want to talk or feed - select feed.
gaming.stackexchange.com/users/30520/Kody Miller
As pointed out by desaivv feeding as a Vampire is not something specific to the Dawnguard expansion, it was possible to both turn into a Vampire and feed on individuals who were asleep in the original game.
However, installing the Dawnguard expansion makes changes to Seduction, resulting in Vampire Seduction allowing you to feed on NPCs no matter if they're asleep or not.
gaming.stackexchange.com/users/52800/kalina
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.
gaming.stackexchange.com/users/41804/vince
I believe you are asking how to feed on people in the Vampire Lord form in order to get "essence" towards your vampire perks.
The way to do this is to hold the attack button (the way you would do a power attack) when you know the next hit will kill them (this is in melee mode).
This usually works when the person falls to their knees and begs for mercy.
It is much easier to absorb essence in caster mode by shooting your red life drain spell at opponents in blood magic mode.
gaming.stackexchange.com/users/41415/LiteshotV3
For a quest, I have to seduce Dexion Evicus then to feed on him. I succeeded in the first task, but the second is becoming annoying: I've reduced his life-total many times to zero, but I don't get the option to feed on him.   So - while I'm in the Vampire Lord form, how I can I feed on him?  I accidentally fed on a previous thrall, but I just can't reproduce that.
How do I feed on people as a vampire or vampire lord?
gaming.stackexchange.com
<the-elder-scrolls-5-skyrim>
gaming.stackexchange.com/questions/79543
gaming.stackexchange.com/users/25745/Olivier Grégoire
You have to wait till he is sleeping, then g o into sneak mode. There are many ways to feed as a vampire in Skyrim, such as feed on people while they are asleep.
The community is giving details regarding the actions required to feed as a vampire.