This is just to inform other travellers: many agencies offer sleeper buses from Vang Vieng to Luang Prabang for 130.000 KIP p.p. The bus should leave between 20.00-21.00 (depends on the agency). This turned out to be a scam.
We had been on sleeper buses before, very comfortable! Instead, agencies in Vang Vieng pick travellers up from their hotel, bring them by tuk-tuk to the main road. Then they hush you into minivans, saying it's a 5 minute ride to the bus station. Before you know it, you're on the highway to Luang Prabang. Our driver repeatedly said the sleeper bus from Vientiane forgot to stop and that in 30 min., we'll be at the sleeper bus (complete nonsense of course). I later heard from other travellers that sleeper buses from Vientiane don't stop in Vang Vieng, and are usually full.
In a sleeper bus, you can sleep and you have a toilet. Our minivan was small and we could not use toilets on the way, since places along the road were closed at night. Our driver also stopped for an hour nap on the way.
For a minivan during the day, you're supposed to pay 100.000 Kip. We paid 30.000 kip too much for nothing + a bunch of lies. When we arrived we demanded 30,000 kip p.p. back. The driver became quite aggressive, but did give us the money back.
This is our third scam with bus agencies in Laos so far (in 2 weeks time), so be warned! Better buy directly at the bus station - we are doing this now, and no problems so far!