Most of the Laos territory is mountainous, but those who enjoy climbing can satisfy their desire in Vang Vieng. This is the only place where climbing is organized, and several agencies offer relative services.
Vang Vieng is located in Vientiane Province, and the Karst hills are probably the most notable in the area. There are about 25 climbing routes, which vary in difficulty from 5a to 8a+. Some have opportunities for a quick and easy hike and are located closer to the Nam Somg river. If you travel further away, you can find longer and steeper routes.
Definitely, the best time for climbing in Laos is when hills are dry, and no rains are expected, which is from November until February. This is also a very hot season, so a hat, sunscreen, and mosquito repellent will certainly come in handy.