I've heard many people like to use plastic newspaper bags or grocery bags as a vapor barrier when cold weather cycling. I'm already wearing winter cycling boots which prevent outside water infiltration, but I've been considering this for really cold days or longer rides below freezing since my insoles eventually get soaked with sweat eventually and become very good at transferring the cold from the bottoms of my shoe to my feet.
What is the most effective way to use vapor barriers inside shoes to keep warm? Should I wear the vapor barrier over my socks, under my socks, or between two layers? What is the theory behind it?