Getting a Vietnam visa in Bangkok is pretty easy. Get a taxi or Skytrain to Wireless Road (Soi Wittayu) in the centre of Bangkok. On the Skytrain you get off at the Phloen Chit stop. Walk out through the elevated skywalk and then down the stairs onto Wireless Road going towards All Seasons Place, which is a huge mall and office block. (There is a big map on a board in the BTS station - check it before you go out through the exits to ensure you know where you're going).
http://www.myvietnamvisa.com/
