Hi Mat, I did this trip 6 years ago in a mini-bus and really enjoyed the mountain scenery, however the roads were slow and not in great condition. I'm not sure if things have improved since then, but even if they have, it's never going to be a highway through that remote region!
We took a bus from Vientiane to Vang Vieng and that was enough travelling for one day. I know that Vang Vieng has become some kind of backpacker party mecca, but there are some beautiful places around there and I'm sure it's still possible to find a quiet place to stay on the river. (We'd planned to stay for 2 nights and ended up there for nearly a week.) Then we took another trip from Vang Vieng to Luang Prabang - again that was enough travelling for one day.
Google Maps says that the trip is 370km's and takes 4 hrs and 47 minutes! So not true! My memory is that both legs took the better part of a day so I would really recommend splitting the trip up. Hope this helps. about 8 years ago