So I've always been intrigued by VBL's, but don't own one and never tried one so all I've got is book learnin' and no real world experience, and I stand to be corrected ...
It's my understanding that one of the most important considerations when using a VBL, is that even if you like to sleep toasty warm, you need to actively regulate your system so that you sleep cool.
The VBL is designed to regulate the small amount of moisture your body passively gives off, by creating a microclimate such that the water content in the airspace between the VBL and your body, equals the moisture level your body is giving off. When this balance is reached, your body quits shedding this moisture, (picture osmosis) and you sleep in a comfortable environment.
If you start getting warm and fail to recognize this and shed some layers of insulation in time, your body actively tries to cool itself by sweating. If you start to sweat in your VBL, you mess up this balance and you end up wet and shivering.
The traditional advice of eating hearty and warm up with exercise before retiring for the night is contraindicated for VBL use.
Starting the night with a dry non-cotton sleepset and sleeping cool seems to be the key to success with VBL's.