I am personally of a mind that if you use rigid foam that you don't need a vapor barrier. Vapor barrier is only needed when using types of insulation where the air and its moisture content can actually migrate THROUGH the insulation and reach the cold side and condense. This happens easily with fiberglass but it can't happen with rigid foam. You might consider taping or spray foam sealing any gaps at the joints and sleepers on the top side to stop air from sneaking through to the cold side.
Some people use plastic under the foam insulation. I don't see any need for it, especially since basements are so dry in my area. Definitely use P/T sleepers. The most you should need it 2x4's on edge because 2 or 3 inches of foam is plenty of r value.