I was in Laos a few weeks ago. First piece of advice would be to reconsider Vientiane. It doesn't offer much, I found most people skipped it or went just to get their Vietnam visas sorted out. Don't listen to whoever said to skip Luang Prabang, that's craziness! Sure it might be a little more expensive.. But that's still cheap. Vang vieng is nice as well but mostly for watching Friends and the now extinct tubing. Although there are some nice lagoons and caves. I would recommend you keep LP and VV, but look into some spots up north of Luang Prabang. Pakse and 4000 islands are awesome but you don't have enough time to travel that far I would say.