Why did you buy warped boards, use roofing cap nails or screw and
washer if needed glue. Drywall over foam, foam is not a good support it
gives, if you want a good job and are taping you need solid support to
wood like 1x2 strips nailed to plywood and foam cut to insert inbetween
the furring strips. The foam itself is a vapor barrier even wthout the
foil.