Lao Aviation flies to Luang Prabang from Vientiane, Houay Xai, Phonesavanh, Luang Namtha and Oudomxai.
Slow boats travel between Vientiane and Luang Prabang and between Luang Prabang and Houay Xai. Taking a slow boat is an adventure for seasoned travellers only. Expect no toilets, no comfort and expect to take your time. This is the real way to see old Indochina.
Speed boats run on the same routes but will take just a few hours - you will pay accordingly. The journey is fast and furious and not for the faint of heart.