The whole point of a VBL is so that it doesn't breathe. If I was going to quit just because I got sweaty, I wouldn't be considering using a VBL at all. My biggest concern is to make sure any sweat doesn't somehow find its way into my sleeping bag, but as long as I tweak my venting and/or layering correctly, that won't be a problem. Besides, even if the VBL turns into a disaster, the rest of my sleeping arrangement will be able to handle all but the worst of storms. I'm aiming at being comfortable in conditions below -40F for several consecutive days--basically, blizzard conditions. Well, as comfortable as one can be anyway....I still need to arrange how I'll breathe and hydrate so I can stay in the sack all night and not ice up the area around my head.