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