I can't speak for Vientiane, but in Luang Prabang the station is further away. However, if you download / install Loca - it's not THAT much further away than the airport.

BTW. Neither the border station nor the one in LP had ATMs that I saw. However, people happily take your USD if you have it (crisp bills only) albeit at rates that are not always favorable. Loca - you can pay with a CC.