Vang Vieng is on Highway 13 between Vientiane and Luang Prabang - by bus about 6-8 hours from Luang Prabang and around 2.5 hours from Vientiane.
The centre is located 5 minutes from the bus station. There are no street names in Vang Vieng, so just follow the many signs (photo of the logo) or ask a local.