I actually happen to own a Vibrant so I’ve done this a gagillion, nay, a fafillion times. In short, yes, I know what I’m doing even though I didn’t sleep at a Holiday Inn last night. (For folks looking for instructions on how to remove the back cover of a Samsung Galaxy S Captivate from AT&T, check out my Galaxy S Captivate back cover tutorial instead.)
The Vibrant actually has one of the easier back covers to remove on a phone. First, emulate a bungee jumper by turning the phone down side up.
See that skinny slot in the middle? Insert one of your well manicured nails in there — I’m talking about your fingers and not your toes, of course — and pull outward to pry it open.