Tqt,
Price depends a lot on your location, if you're in a major metro city, then airfare would be relatively decent - here is one
site you can check for deals to VN.
Prices should drop more so in the near future now that the
US-VN Air Agreement is now in effect. So that means more competition. Việt Nam Airlines will service to SFO & LAX. AA has already
setup an office.
Yeah, Hạ Long Bay was a blast - not many places in the world where you can occupy an island all by yourself.