It'll be a long ride and would be best done by breaking it up! From Siem Reap , buses go south as far as either Kampong Cham or Phnom Penh then back up through Stung Treng to the border. This part of the trip took a full day (leaving Siem Reap at 5am, arriving at the border around 5pm). A good idea might be stopping for a couple days in the 4000 Islands, just 30 minutes passed the border in Laos. From there, you can catch overnight sleeper buses from Pakse to Vientiane, maybe 9 hours or so, I forget exactly.
#2 dano_b has been a member since 20/12/2009. Posts: 24
The easiest and quickest way is to go through NE Thailand.
Cross into Thailand at the Poipet/Aranyaprathet crossing. Take the bus from the bus terminal in Aranyaprathet to Korat(Nakon Ratchisima). From Korat you can either take another bus or the train to Nong Khai, and from there cross into Vientiane.
If you leave early enough you could probably do it in a day.
#3 yogakhaolak has been a member since 1/4/2010. Posts: 45