[She still doesn't know? The differences between their worlds and the creation of vampires. He'd done a lot of comparing of worlds when Faith had gotten here, so he assumed Buffy had at least gotten a little through people from her world. But then, Spike seemed afraid to approach her and Faith and her barely said two words.]
As I said. It's supposed to be almost forbidden to get blood from a pureblood. They don't often give out their blood at all. Even to the ones they create.
All it takes is a bite by a pureblood for the transformation to begin. There isn't any draining or exchange of blood involved. All they have to do is get their fangs into you. They don't even really have to feed much.