The calculated distance (air line) between Xam Nua and Vang Vieng is approximately 146 miles respectively 236 kilometers.
By car or train, the actual journey to Vang Vieng is certainly longer, as only the direct route (as the crow flies) between Xam Nua and Vang Vieng has been calculated here.
Air line (approximately)
236 kilometers
127 nautical miles