I think in the south and north there are different concerns. Up north we have to worry about frost heaves and winter stuff, so I think we tend to insulate around and under our concrete. At least we did on our house build last year. In the south Im guessing there are different things to combat, which might be why they do vapor barrier. I know when I lived in NC, encapsulating crawl spaces (basically sealing them from top to bottom with vapor barrier and some insulation) was a big business because of all the humidity. I'm guessing this is similar.