The calculated distance (air line) between Luang Namtha and Vang Vieng is approximately 156 miles respectively 250 kilometers.
By car or train, the actual journey to Vang Vieng is certainly longer, as only the direct route (as the crow flies) between Luang Namtha and Vang Vieng has been calculated here.
Air line (approximately)
250 kilometers
135 nautical miles