Vapor barrier question

I scored a free compact upright freezer:rockin:. Yes, it is only a freezer. It is somewhere in the 4.0 cubic foot range (32" high by 22" deep and wide). Anyway, I am planning on building a fermentation chamber similar to the many I have read about the past few days. I want to use the freezer part during the summer months, and I will use some sort of heat source for fermenting in the winter. So finally to my question, should I use a vapor barrier during construction, and if so, where should it go?