Buffy Vamps seem to be loosely based on Vampires in 'The Masquerade', although without the clan structure of the Vampires in that mythos.
The logic (liberally lifted from Anne Rice) is that Vampires get harder the older they get, but also have a basic strength based upon how many generations of Vampires seperate them from the original Vampires (Cain and Lilith are faves for this role).
This is used in films like 'Blade', where the 12/13th Gen trailer trash vampires are really, really quite pathetic, but where an 8th generation vampire is really, really bad news.
Has anyone played 'The Masquerade'? It's a live role playing system... I did it with a old gf of mine a few times, it's quite a laugh; apparently it's pretty popular in some American and Canadian cities, where there are groups playing Vampires, Slayers, Techo-Mages (don;t even ask) and Warewolves, all interacting together, or more accurately, trying to kill each other. The Vamps have the best fun of course, as they try to kill each other in a perpetual struggle between the clans, and there are 'bad guy' vampires too, who don't respect 'masquerade' a centuries old compact which means that the existence of vampires is kept a secret from the world.
It's a bit like (i.e. similar but way more complex) 'Killer' a live role playing game I introduced in my High School. You basically try to 'kill' fellow players, last one standing wins... very amusing to have to run down a packed corridor from someone waving a bannana who's trying to get within the 2 metre effective range of a bannana. Most people don't know about the game, so their reactions are great... Other 'weapons' were water ballons and pistols, confetee, water, toy guns, ping-pong ball bazookas... rulers, handfulls of super rubber balls.