I use to be somewhat skeptical of VBL in general. I've come to consider that they have their place as a niche piece of gear for certain conditions. Particularly bitter cold and being out longer. Too many people have reported that they help a lot in those conditions.
I'm still skeptical of the active use of VBL's, ala Skurka's reports for example. Having used VBL in stationary/rest conditions, i could only imagine the difficulty in regulating it during physical exertion. I mean, i understand the point, but i prefer breathability verse being able to wear a lightish down jacket. In that case, i would rather not save weight, but instead save comfort.
Meanwhile your baselayer is constantly soaked, no matter how well you try to regulate it.
When you're sleeping and your metabolism goes way down, i think the main regulation you really have to do, is to not have a bag or quilt that is too warm for the conditions. Figure the VBL will add at least 5 degrees of warmth to your sleep system.
Personally i would not use one, since some of my past experiences with same, until it gets down to around 15*F or so. Around low to mid 20's, was a bit clammy at first, which i didn't like the feeling of. My body did seem to adjust though, and when i woke up, the clamminess had gone down. Some say one's body does adjust accordingly some to the sensing of perspiration on skin. I'm not sure, it was either that, or just that my metabolism had gone down some more notches by that point.
Matt, just curious, have you experimented with using VBL's?