Its a long trip but not so bad. Took us 23hrs then all 11 Europeans on the bus (only a 22 seater, not a sleeper bus) hired a songthaew to go straight through to Vang Vieng.
We took the scientific approach of going to a few places selling bus tickets and finally chose the cheapest combined with the best looking photo of a bus. Didnt really matter as it turns out we all paid varying prices at different places. And the photo of the bus was nothing like the bus that took us as far as Vinh to get the smaller one. And we all took different buses to get to Vinh as well.
As for safety, I wouldnt worry,, just do the usual and keep your valuables with you.