It is more important to place a moisture barrier on the dirt floor than to place a vapor barrier over the insulation. Plastic sheeting should be placed across the dirt floor of the crawl space and fiberglass insulation between the floor joists above. Under normal circumstances, a vapor barrier should be placed on the warm side of the insulation, but it is a very difficult job to undertake in such a small space. In your case, the floor above can act as the vapor retarder. Make sure that any small holes in the floor above are sealed completely, and if you have linoleum or vinyl flooring on the plywood, this will provide further vapor protection.