Thakek is the new spot to be in South East Asia. Greenclimbers has established 2 small and nice camps about 12km east of the small border town of Thakek offering accomodation and food and of course a lot of climbing. Currently there are about 300 routes, mostly single pitch sport climbing, on excellent limestone. Routes range from easy grades for beginners up to about 8b. You can find everything from technical face climbing to horizontal roofs and crazy tufa formations. All routes were developed in the last few years and bolting and access to the sectors are excellent and well maintained. Be sure to get the latest guide book "Climbing in Laos" that also covers most other areas in Laos as the area is still expanding with lots of bolting going on. There is more than enough to do for many weeks and on rest days you might enjoy the many caves in the area, just relax next to the river or do your visa run.
Pha Tang is about 30km north of Van Vieng on the way from Luang Prabang to Thakek. The climbing spot offers a good number of long routes on excellent limestone, mostly sport climbing but also some multi pitch routes. As this spot is not frequented a lot some of the routes require cleaning from dust and spider webs ;-). About 4km south is a small guest house at the river offering food and accomodation, other than that there is nothing much. The spot is definitely worth a visit when you pass there but requires that you have some sort of transport. The climbing is in the shade all day. Bring moskito coils.