Theres a pretty good article by Eddie Meechan in  this months TGO, I'm not all that "clued up" myself on this. I think its more used as a sleep system due to the lack of activity. In fact, as I think about it more, it seems to me that if you are active (i.e. cycling) and therefore producing sweat at a greater rate then it wouldnt work.

I think it works on the principle that there two diff kinds of sweat, one that your skin does to lubricate it and stop it cracking which will stop when the skin is moist. The other is the cooling mechanism which will keep working until you are cool. If you are sleeping then there's no activity so it should be the first type of sweat which will be held next to the skin inside the vapour barrier and so then not disappear into bedding causing your body to keep producing it untill your bedding is damp. I'm not sure as to how the VPL keeps you warmer though, I'm gonna re read that article again when I get home in the morning.

Maybe the super brains of John Burley, Captain Paranoia and the like could explain the principles of it a little better?