I went to Laos for a couple weeks in February and I really enjoyed it. Took a slow boat down the Mekong from Huay Xai (border near Chaing Rai and Chiang Mai, Thailand) to Luang Prabang. The roads aren't great there.. it takes like 6 hours to go a couple hundred km, but that was part of the fun for me. Learned a lot at the Plain of Jars, especially Vietnam War history. Things they don't teach in school here. There were remains of old bombs everywhere. In the distance we could hear the government destroying mines and bombs that are still left unexploded from the war.
Vang Vieng has some nice scenery. The government has cracked down on the crazy party tubing of years past, but you can still go tubing or kayaking and have some drinks. Lots of caves to explore and rock climbing/caving in the area. I didn't go to Vientiane, but I went south to Pakse and rented a motorbike for a day to check out the Bolaven Plateau. There were some nice waterfalls in that area. My last stop was the 4000 islands where I basically relaxed in a hammock for 4 days watching sunsets on the Mekong. After that I crossed into Cambodia. I would definitely go back, but probably in the wetter season when the rice fields are green. It was pretty dry in February.