Originally Posted by
mightbeintangiers
then forget the visa. Noi Bai airport in Hanoi is approx 45 minutes from the city. There is little to nothing near the airport to see. The best you'll get is a taxi ride through a little countryside on a highway and then back to the airport. If you do have more time, of course Vietnam is always worth a visit. Best of luck on your trip.
The visa is definitely necessary for this transit, especially if flying AirAsia.
JettaTurbo, make sure you get a multiple-entry rather than a double-entry visa. It doesn't cost much more and you'll be able to go back to Vietnam to see that place properly at some point in the near future (until the expiration date of the visa).