@Mortein: They weren't lighspeed at the beginning of Z, but I can see them reaching relativistic speeds towards the end of the Cell saga and beginning of the Buu saga.
" Vulcan sucks out all of Vegeta's energy in about 3sec and kicks his head off. "Vegeta wouldn't sit around waiting for him to drain him of energy. And Vulcan has showed he can't take a punch to save his life. Vegeta could make this physical, in which case I don't see Vulcan winning. But if he just chucks around ki blasts, then yea I see your scenario happening.