Note: I have not purchased Dawnguard, but I plan to, so for the love of god, don't talk about anything not related directly to my question.

Anyway, I was wondering: approximately how many kills do you have to get with werewolves and vampire lords in order to get a perk?

The number of kills required per perk increases with each perk gained. However, it's really not that many. One good rampage across Skyrim will get you the whole tree, easily.

cool, thanks.