Please, check on nexus:

http://www.nexusmods.com/skyrim/mods/49038

WORKS WITHOUT SKSE
COMPATIBLE WITH ALL MODS

Every vampire NPC is able to feed on characters just like the player

- He will automatically pick up the victim at night and inside the interior (preferring dark areas). The stealth skill affects the chance of detection. Characters with high moral (like Serana) will not feed on their friends.
- Same scheme with sleeping characters, but the chance of detection is lower
- Vampires are able to feed on their enemies in combat (if the vampire's health and stamina are higher). NOT A KILLMOVE, this one is just a soft bite!!! The enemy will survive this and won't take any damage.

BUT... if the vampire's level is much higher, he can restore his health. Such mechanics makes the vampire encounters more interesting and challenging.

Of course, vampires will never feed on other vampires or undead.

Videos:

http://www.youtube.com/watch?v=hX8oYjIPrH4

http://www.youtube.com/watch?v=5tQdOduLI5M

There are also reports of vampires being able to feed on your character too if your not chareful