You get the Villain tag when you use Source Vampirism three times, excluding the god.
Can not confirm. I have definitely used source vampirism more than three times, possibly 10 times or so, and haven't got the villain tag (yet?). So it's either a bug or there's more to it. =/
(Wait, do you have to use source vampirism on living targets? I usually sucked on ghosts or corpses.)
