Tham Poukham

Tham Poukham - Blue Lagoon, 7 km west from Vang Vieng town; accessible by mountain bike or motorbike. In Vang Vieng you can rent bicycles with maps provided, Be careful along the way, in recent years a number of imposters have shown up, all claiming to be the 'blue lagoon'. Keep to the main road and you should be OK). 8AM-6PM. 

A spring fed lagoon at the bottom of "Golden Cave". The lagoon itself is small, and surrounded by a parking lot, vendors, and construction. On most days, it is plenty crowded, with tourists practically landing on top of each other from the rope swing. Despite being crowded its beautiful and definitely worth a visit; especially to cool off. The waters are inhabited with a few hundred carp that will eat locally sold fish food out of your hand. The cave above requires a modest 100 m climb up a makeshift bamboo ladder. Once inside, there is a short walk to the Sleeping Golden Buddha and glimmering stalactites about 300 m further inside. 10,000 kip entry and 10,000 kip to rent a head-torch, mandatory if you go deeper into the cave than the Buddha (check the battery in advance). Guides for the cave advertised at 50,000 kip but this is probably negotiable. Guides are probably not necessary since the path inside is well worn and marked in spots.

Source http://wikitravel.org/en/Vang_Vieng