The online site for Vietnam is quick and easy (and legit!); check Lonely Planet for the recommended one, I don't recall the exact name off the top of my head.
Don't forget to bring a passport type photo with you on the flight to give them on arrival; if you do forget (as I did) they take one for you and charge $5. Can't really complain as the fault was mine in the first place, but when you see you can have a fine meal and a beer for the same money it's a bit steep!