The thing with Vibrant is that it works with ATT due to 1900 MHZ band compatibility. But in some areas you might get edge. In my hometown Denver (Strong ATT network) I tried vibrant with my ATT sim and got only EDGE. But in California vibrant was able to get ATT 3g network.
Regarding price, once again using Denver as reference. You can get one for 185$ new one or used in mint condition. So why don't you get it when you go to US? If you are going to US, then you might as well as buy one of these phones at good prices. These prices are craigslist prices as they are more popular in US to shop for phones without contract. They are:
$ 185 - 200 - Vibrant, Captivate
$ 250 - 300 - G2, MyTouch 4g
$ 300 - 350 - Inspire 4g, Atrix, Nexus S, Sensation, G2X, Infuse
$ 350 - 400 - MyTouch 4g Slide (Dual Core), EVO 3D (CDMA)
How do I know this? I have a friend in Denver that has a cell phone shop. So I get the prices from him as to how the phones (Used) is priced.