There should be some discussions about vbl in the archives. Stephenson (not Bill, but the tentmaker) is a big proponent of vbl and there is a fair amount of into on using it on their website. They sell vbl clothes and bag liners.
http://www.warmlite.com/start.htm#anchor28960

My understanding is that vbl is for use in very cold, very dry weather and the purpose is to keep moisture in so you don't chill from evaporation. It also keeps your bag from getting wet from sweating at night, your basic evaporation into the bag. Read Stephenson's lengthy explanation on their website.

It must work, but the idea of waking up wet, just doesn't appeal to me. Then again, I'm not out in -40C weather either, so maybe then, I'd feel differently (no pun intended). <img src="/forums/images/graemlins/wink.gif" alt="" />