NEF, I don't think you want that second layer of vapor barrier. Is your floor damp? If not, just put down pressure treated 2x4s and use a good foam insulation between them. Then cover with 3/4 plywood. The vapor barrier is to protect the insulation. A good foam is not affected by condensation.

Another thought - I haven't researched this but have you looked into a heated floor? I've seen shows where hot water is run through pipes in the floor to provide warmth.

If you are interested in heated floors, look at the site: Warmfloorcenter.com for information. Rodney sells and installs radiant flooring systems. His number is 1-800-562-8036 if you much rather talk to him directly.