First, you need to get some special screws or nails that have a large
plastic washer to prevent them from being pulled through the
insulation. You should be able to find where you bought the
insulation. Glue is a fine idea, but the nails will help hold it in
place. As for the vapor barrier, since the foam is a close-cell foam,
it acts as a single barrier. The fact that it's not a breathable foam
makes it a good insulator.