Well, as others have said, vampires are just one component of Buffy's slaying activities. A big part, sure, but just one.
As for sacrificing Angel and Spike to terminate every vampire in the world? Would she do it? You know, I'm not entirely sure. Buffy's "Greater Good" decisions tend to fluctuate pretty frequently. She kills Angel (or thinks she does) to save the world in S2, then won't even contemplate sacrificing Dawn to stop Glory and, literally, save all of reality, including Dawn, from imminent death. Despite her powers, abilities and capacity to help people, she wants to die for the bulk of S6 because of her own feelings of emptiness. Then in S7 she says she would be prepared to kill Dawn if she could do S5 over again while at the same time resists everyone's advice to eliminate Spike (which, I don't care what anyone says, I felt was the right decision) when it appears he's gone rogue.
So would Buffy do it? I guess it would depend largely on her frame of mind at the time. SHOULD she do it?
No. No she shouldn't.
It's virtually impossible to truly and accurately weigh the figures of which would tilt the cosmic scale of greater good more, but the way I see it, the entire planet, Buffy included, would have been enslaved by Jasmine had it not been for Angel. Without Spike, the Turok-Han would have slaughtered Buffy and all the Scoobies and steamrolled through Southern California before the U.S. military could have gotten a handle on them. How many people were saved because of those two acts alone? Over the course of both series' he and Spike repeatedly prove themselves to be heroes of the first degree. They're both champions who save lives and the world just as effectively as Buffy does.
