I've been to all the places in Laos you're talking about, and also to Vietnam... pretty much covered distance from Hanoi to Hoi An. Loved Luang Prabang and I might give Vang Vieng another shot, even just for some bike riding or something like that.
My fiance has suggested that we should go to Saigon, Nha Trang and Dalat next time we get back to Vietnam as he has covered it down there, and I haven't. The problem however is that I also want to go to Cambodia and China, so we really need to decide what the hell were doing and stick with it. I'm also seriously considering Burma and maybe some beach trip for a few days, or Chang Mai.
My fiance has suggested that we should go to Saigon, Nha Trang and Dalat next time we get back to Vietnam as he has covered it down there, and I haven't. The problem however is that I also want to go to Cambodia and China, so we really need to decide what the hell were doing and stick with it. I'm also seriously considering Burma and maybe some beach trip for a few days, or Chang Mai.

Sihanoukville it's the perfect place for me. Lots of nice beaches full of beach shacks and bars, day trips and activities if you are interested, great food, cheap beer (25 cents!) and a good nightlife ranging from pubs to beach raves to live music bars to casinos. I'd go back there in a second. 