Regrettably, I think Blade and Buffy would spare the Cullens owing to their strong ethics. Even Blade had to choose to team with vampires in Blade 2...it's a matter of these particular slayers thinking ahead tactically and strategically.
The senior Cullen...I'd have to spare owing to him being an MD. Professional courtesy, of course. The rest of 'em...
Hell, what you need is to either drop a strategic nuke on Forks...or let loose a Terminator on them. Yeah, it's a genre mismatch, but can you imagine Summer Glau laying waste to the Cullens with a Mk.19 automatic grenade launcher? That sucker will blow them apart! If these vampires have to be cut to pieces first, that's the way to do it!
And if the sequel ruins werewolves...I am going to start tearing out throats. Pack signups start after the first sign of crap from the sequel.