That is not a lot of time as I can see you are aware. My suggestion is to pick one town and focus on it. If you
have to go via Vientiane do so AQAP as there is relatively little to see there compared to regional towns.
I've only been to Laos once, but it was in 1995 and we had a week. I think we spent 4 days in Luang Prabang (then like the end of the world), and 2+ transiting via LP. Options to travel further were very limited for a time-poor traveler.
Van Vieng looks really nice culturally, but perhaps rather hipster-tripster from a quick read of the Wiki page. 4hrs to Vientiane but 8hrs to LP (or vice versa), that's a real time-stealer as they will likely be daytime buses.
http://en.wikipedia.org/wiki/Vang_Vieng
Maybe define what you want and work backwards. i.e. do you want to hang out with a bunch of stoned antipodean kids doing bungee and zip-wire rides?
Happy to field other questions as best I can.
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