Small mod that opens up the possibilities for feeding as a vampire, without needing a big overhaul. To make this super simple, I didn't add new abilities to your character to clutter your menus, instead: Just activate your victim while having weapons/magic/fists unsheathed.

Vampire Feeding Tweaks

3 New Types of Feeding:

• Feed on almost defeated enemies: If you are a vampire, and an enemy NPC is in kneeling in combat (bleeding out stage), you can feed on them.
  

How to: Activate a kneeling enemy while you have your weapon in hand.

• Feed on unaware enemies: If you are behind an unaware enemy, you have a chance to feed on them. There's however 10% chance they resist the attack.
  

How to: Sneak upon an enemy, press shift and activate them. This does 150HP damage and will make the actor start combat with you.

• Feed on fallen enemies: If you have recently killed an enemy (less than 1h in-game since death), you can atempt to feed on their corpse. There's a 20% chance the victim still has enough blood to count as "feeding". If not successful, you will only take a "sip", restoring 25HP.
  

How to: Hold Shift + Activate a recently killed NPC.

Press SHIFT to dynamically change the interaction

Compatibility: I don't edit anything from vanilla or other mods. This means other vampire mods will think you just fed on a sleeping victim. This should be great for compatibility, as I imagine most vampire mods just hook on the vanilla scripts. Keep in mind though that many vampire overhauls have similar feeding features, so I'm not sure if it makes sense to install this on top. If you do install it with an overhaul, feel free to let me know how it works in the comments. 

Who is this thought for: Those that enjoy vanilla vampires but think feeding only on sleeping enemies is a bit too boring. 

Requiem: Only mod I will offer a compatibility patch for is Requiem, once it launches on Special Edition. Specifically to requiem: Since I'm not touching any records or functionalities from vanilla, the only thing that needs to be changed here is removing the "blood potions" on enemies that Requiem adds, since you now have a proper "feeding on corpses" animation to replace them.

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Want more? This is part of my small vampire tweaks series of mods:

• Vampire Lords Can Fly (With Collision) - Make use of your vampire lord wings and fly away! No scripts, no collision issues, very simple approach
• Vampire Feeding Tweaks - Tired of feeding only on those that are sleeping? Feed on kneeling enemies, unaware victims or recently killed enemies
• Sun Affects NPC Vampires - Why is the player the only vampire being affected by the sun?