The bus journey from Vientiane takes about 10 hrs, the road is good and some impressive scenery along the way.
Vang Vieng is 2-3hrs from Vientiane, worth a stopover. Plenty of nice, cheap guesthouses for the backpacker, open air restaurants/beergardens with local and travellers' food, where one may also acquire officially banned stimulants.
There are treks into the nearby mountains on offer, some lime-caves to be visited etc. I didn't do any of that, but enjoyed floating in the river with a hired inflatable:
There's a typical town market where we bought a few bbq sticks: