I used vapor barrier socks (neoprene) to good effect when I spent 30 days in the field continuously in the Cascades, mostly above tree line. The idea of vapor

I used vapor barrier socks (neoprene) to good effect when I spent 30 days in the field continuously in the Cascades, mostly above tree line. The idea of vapor barrier is that it goes directly against your skin, underneath the rest of your clothes (or in my case, socks), and it blocks all moisture from getting from your skin into your clothes. This keeps the rest of your clothes dry and therefore warm. It's pretty specific to winter conditions. The theory is that once the humidity inside the vapor barrier gets super high, your skin will adjust and stop perspiring. What this means, though, is that sweat gets trapped against your skin for hours and hours, so taking off the vapor barrier releases a lot of stink. And you do have to take off the vapor barrier when you can, to allow your skin to dry out and recover. I wore thick wool socks over my neoprenes when hiking/climbing, and then changed into wool camp socks (without neoprene) when we got to camp and set up tents, so that my feet could air out and dry out. It kept my feet warm and happy in some pretty crappy conditions.