Not sure I believe the sensible sweat thing, but I believe the frost in the bag at subzero temps. I wish I could sleep in a vbl but can't stand it. my solution has been to use a synthetic outerbag so the frost layer stays in that instead of my down at very cold temps (or have a wood heater in my tent)

I know enough people who use vbl to know it does work. the moisture stays in you or goes out through the top.