There is no boat trip organized, especially with western standards. You might find boat going up and down the Mekong, but not all the way long. there are rapids on the river downstream Luang Prabang and they are building dams on the river ... So no way ... I did the trip from Xayaburi to Vientiane few years ago ... Nice but quite slow and noisy but much better than the bus. about 6 years ago
Agree there is no organised boat from Luang Prabang to Vientiane. You can get boats that travel from town to town but only in the right season. In the dry the river can be too low and there are lots of rocks and in the wet the river can have too many rapids so boats won't go during either of these times. It is supposed to take around 5 days when you can go but expect this could string out to longer so you would need to have plenty of time. If you are a tourist and strapped for time you wouldn't do this.
There are organized boat trips (on a slow boat) that take two days from Luang Prabang to the Thai border as well as a few speed boats that take 5 hours (very uncomfortable and can be dangerous) but there are also plenty of private slow boat trips you can take around Luang Prabang to villages on the Mekong and also on other more beautiful rivers around Luang Prabang such as the Nam Ou so if its boating you are after then slow boats around Luang Prabang and northern Laos are much more spectacular. You would hire your own private boat or join with some others and can enjoy 1 - 3 days on beautiful local rivers with high mountain escarpments.
We have our own charity association in Luang Prabang (www.globalsupportassociation.com) and we often take boats to deliver clothes or school books and supplies or collect people needing medical treatment to and from needy villages which have no road access. We always enjoy the beauty of Laos on those boat trips.
The road trip from Luang Prabang to Vientiane has spectacular mountain scenery. It is only 300 or so kilometres but can take 8 - 10 hours to travel. Plane takes only 40 minutes or so. The road trip is the best but we always hire our own vehicle instead of the bus as there is much to see on the way and we usually make 2 days out of the trip to make some stops at Nam Ngum and the Nam Song in Vang Vieng. The bus rarely stops so not very relaxing. Hope this helps. almost 5 years ago