Seeing as Savannaket wasn't the most exciting place ever we took the first bus out in the morning. Actually, we skipped that one, it left at 6:40 but the 8:15 bus did the trick as well and seeing as Vientiane was only 400 kms further up the road, how long can it take?
The answer: Apparently 10 hours. Locals have to get on and off the bus all the time, the bus-driver needs a break every 2 hours, etc. A tuctuc-ride later we found our hotel in Vientiane then had a drink and some food at the river and passed out early.