The best time to visit Vang Vieng is the cool dry season from November to February. Temperatures are comfortable, the skies are clear, and the rivers are low enough to make tubing pleasant rather than dangerous. The karst scenery is at its most dramatic in morning mist.
March and April get hot — over 35°C is normal — and dusty. The rains arrive in May and last through October; the landscape turns intensely green and the caves flood, closing some entirely. The river runs fast and brown during the monsoon. If you visit in the wet season, swimming in the blue lagoons is still possible but check cave conditions locally before heading out.