we did pretty much exactly as Rufus suggests on that same route in 2006 and it worked well. be sure to get to the bus station in luang prabang early to get a seat. the extra money for the VIP bus from LP to VV is really worth it too. vientiane is a nice place to visit, so plan to spend some time there if you have any to spare. nong khai, back on the thai side, is fun too, and the best place to catch the overnight train back to bangers, if you choose to go that way. i'd make my Thai train reservations as soon as possible. have fun.