In one episode, Buffy was able to bend the barrels of a werewolf hunter's rifle without straining, and there was that time when she lifted that I-Beam when she was working on Xander's construction crew, so it's safe to say that her strength is far off the scale s far as what any human should be capable of, but she's never tossed a car at anything, so probably it's not up there with Wonder Woman. For argument, let's say she's 5 times as strong as any other human. In "Welcome To The Hellmouth," Luke says that he is stronger than she is, so let's say that the Slayer is as strong as the AVERAGE vampire, but some, (Luke, Kokistos, etc.) have her beaten in the sheer brawn department.
A couple of times, Angel has ben shown moving so quickly that he was just suddenly somewhere in one instant and somewhere else within 20 feet or so seemingly instsntly. I can't ever remember ever seeing Buffy do that, but then, I've never seen any other vampire do it either, so maybe it's a trick he picked up somewhere like Dracula's being able to shape shift (let's hope so, because if all vampires were that fast, the Slayer would be sausage before she could swing the first punch.). On the other hand, Angel doesn't move that fast very often, so maybe even he can only do it under certain circumstances.
Let's say that on a scale of 1 to 10, the average vampire's speed is about a 5 and just for the sake of argument that Buffy's is a 6 0r 7, which would make sense, because her superior speed would then be be the deciding factor in most fights.
So Buffy is as strong as the average vampire, but there are some stronger than she is, and faster than the average vampire, but there are probably at least a few faster. If she were stronger and faster than any vampire, and her strength and speed were typical of all Slayers, then Slayers would be virtually invincible.
So it's safe to say that both Slayers AND vampires are far stronger than normal humans, and the Slayer's strength is about equal to the average vampire's, while her speed is probably a bit greater. I don't think we could say, however that Buffy or any other Slayer, is stronger and faster than all vampires, because the series has shown that to not be true. We could say, however, that the Slayer's FIGHTING SKILLS, at least as her experience increases, is greater than most of the vampires that she's likely to encounter, which is probably for the best, otherwise the show would get boring very quickly. So in the match up of Slayer vs vampire. the Slayer would be more accurately compared to Batman than Superman. Her superiority comes from a combination of speed and skill, not sheer muscle.