This is not the first time that there is a vampire plot in the game. Sad, but true. IIRC those previous attempts were not supported.
My question is, then: Has the playerbase told about this fact to those playing that plot? And if so, what has been their reaction?
I do not think the problem is to deal with this new "vampire hunter" guild, but on the origin, the ones that chose, yet again, to bring vampires in Planeshift.
I do not see that need some players have to use outsetting creatures to develop their plots of evilness, as if they require a power that they can't have using other ways (vampires are, from a setting point of view, as extraterrestrians would be to us) I honestly think there are other ways to act, and, despite I haven't seen this ones acting, I think that the best thing to do is inform them that what they're doing has been attempted many times before, without positive success, and not because there were no abilities to roleplay such a char, but simply because the char is not roleplayable in this game.