The calculated distance (air line) between Luang Prabang and Vang Vieng is approximately 70 miles respectively 112 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 Prabang and Vang Vieng has been calculated here.
Air line (approximately)
112 kilometers
61 nautical miles