I saw a similar problem on another thread (might be a different forum) and I think the problem was the charger. Are you using the charger the came with it?
Although the Vibrant uses the micro usb, I think the wiring is off on their connections. If you use an aftermarket charger, you have problems with the phone.
I tried a cheap charger and had problems so I stopped using it. I bought a few oem usb cables from ebay that were meant for the galaxy s and I have not had the problem since.
My friend has an old motorola flip phone with a micro usb connection. He used a blackberry charger to charge it and it worked, but when he tried my vibrant one, it never worked, so I guess it is just Samsung changing the wiring on the cables.
Not sure this is really the solution, but it worked for me. Hope it helps
If you are using the charger that came with, maybe its a problem with your phone