My experience with using a vapor barrier in a sleeping bag is limited to six below-freezing nights in the Rockies last summer. After the first frosty morning, when my down bag was distinctly damp, I remembered what I'd read about vapor barriers and tried wearing my non-breathable (silnylon) rain jacket and pants to bed over my base layer. It worked just fine. I avoided getting sweaty in the early evening by leaving my sleeping bag open (please remember that this was a Rocky Mountain summer, not a Canadian winter!). Usually after the second time I woke up (one of the problems of aging is that I have to get up several times during the night), I'd zip the bag up. By about 2-3 am, I'd have to snug up the draft collar in my sleeping bag. A couple of nights I was borderline cold about 5-6 am. Those were the nights my dog's water dish froze solid, leaving one puzzled dog in the morning! I didn't sweat and didn't even feel damp. My sleeping bag stayed dry, too. I normally do a lot of sweating, so this was a big surprise to me!