Yes, and they do help. But in fact, keeping the feet warm on a bike is much harder than in climbing unless your in extreme conditions of cold and altitude such as Everest. This is the picture I was trying to paint. With walking the feet get more blood circulation than in cycling. So the vapor barrier by itself only gains you a couple of degrees. Generally not effective by itself without added insulation and wind protection. However at 40F it is enough to get you home in better shape than with nothing. You could also put a small sandwich bag over just the toe of the shoe to block some of the cold wind penetration in an emergency situation in addition to the vapor barrier.

Getting off the bike and walking for 5 minutes will also help a great deal to warm up the foot. IF your not wearing road shoes.