Can’t think of much worse than a vapor barrier between me and the bag. Yuk.
When it's below zero it's critical in a down bag... but you've got to have synthetics on over the skin.
I'm late here, but disagree. I've used quite a few times way below zero with no vapor barrier. I would guess 95% of subzero usage of down bags is without a VB. This by folks who know what they are doing. I have never used a down bag with treated down, or one of the new wonder fabrics like GoreTex. But these barriers are "one-way-permeable", and not vapor barriers.
My experience is finite, but includes temps from -20 to 0 in a dry environment, as well as 0 to 32 in a varied environment. In all cases I was fortunate to keep my bag completely free of atmospheric dampness, so all moisture came from my body only.This is no small thing, as 200 lbs of human looses a lot of vapor. Also, I usually have at least some time to let the bag air or even sit in the sun, even if cold out. I have used vapor barriers on my feet with boots, and they suck maximally, but work. Haven't needed them in a bag.
I agree that a down bag, once wet, is very tough to manage and may be a deal-ender.
