> How "way" are we talking, Roger? My temp range would be about -5C to -20C for my winter backpacking...do I need to go colder for a VB to work?
Well ... I really need a few others to chip in here with advice. The problem is that with any moderate weight SB, you are likely to be warm enough at -5 C, so a VB would make a fair bit of sweat inside.
To make a VB work nicely (I said 'nicely', not 'properly'), you really need to be sleeping pretty cool so you are not giving off much water vapour. I guess it depends on the SB you are using as to how cold it has to be for that to happen. And it is going to depend on what you ate for dinner, how much wind is blowing through your shelter, what you are wearing, the phase of the moon, and ...
Only thing to do is to try it out. Using a VB needs a bit of experience - which usually means doing the wrong thing a few times along the way :-) :-)