Bus routes in Laos?

Hello,

I'm planning a trip to Laos around the beginning of March and would
like to travel via following route:

Vientiane - Vang Vieng
Vang Vieng - Phonsavan
Phonsavan - Luang Prabang
Luang Prabang - Huay Xai

I know that there are several buses a day from Vientiane to Vang
Vieng and Luang Prabang and also it's possible to get from
Luang Prabang to Phonsavan by bus with one change of buses
or so.

Does anyone have any information about buses running between
Vang Vieng and Phonsavan? Of course, going to Luang Prabang
first is a possibility, but that would be extra distance
and also I would have to return to Luang Prabang later to
catch a boat to Huay Xai and the Thai border.

thanks in advance,
Eino