How to use a vapor barrier with cycling shoes for cold weather riding?
I rode a couple winters as a bike messenger in the northeastern US, and this is a classic messenger trick for the worst cold wet days. While your feet may still become somewhat wet, at least they will be warm and wet. I liked wearing a thin sock on the inside, the plastic bag, another pair of socks, then shoes.

This is not really a dry-day technique. Then, as @hillsons said, you will just have wet feet for no reason. When it's dry and really cold, put flat pedals on your bike and wear normal civilian winter footwear. Hiking boots, for example. But when it's 35 degrees and raining and you are silly enough to be on your bike, the plastic bags trick is the way to go.

One place to search advice on vapor barriers is in the camping/backpacking world, there's been plenty of experimentation with them since at least the 60s in that community.