Vang Vieng is situated in the middle of what is technically called a "karst topography." It's strange coming south on Route 13 to Vang Vieng - the hills mutate as you go. You're driving through very hilly terrain on a road that thrashes about like a snake with its head in a trap, admiring the beautiful views. The hills become more and more ... lumpy. Like someone with a really big fist got under the crust of the earth and just started punching upwards. They just look wrong, but in a very attractive way.
Vang Vieng is a very small town that's seen explosive growth since the country opened in 1989, all of the growth based on supporting tourists. Most of the tourists that come here are backpackers, and quite young. In the center of the tourist strip there are a bunch of restaurants that run TVs at a fairly high volume the entire time they're open, and most of them show "Friends." If I never hear the "Friends" theme again, I wouldn't mind at all - and I haven't even been in any of these places. (Keep in mind that over here the vast majority of restaurants and stores are wide open - they have no front wall, you just walk right in.) There are also a few places that have food items prefaced with the word "Happy." This means that there's marijuana or hash cooked right into your meal (never mind that it's illegal). Again, something I've avoided. Fortunately there are several good restaurants without TVs or happy meals.
Popular activities include tubing, kayaking, trekking, rock climbing, biking, and caving. I've been out on a bike, I've gone in caves, and I've been kayaking. It's a beautiful area.