I am putting down 3/4" oak plywood over concrete. The plywood will be attached to 2x4s on edge to allow for wiring and small duct work. The concrete floor has 3/4" foam underneath. Do I need an additional vapor barrier? As the 2x4 rest on the concrete can I use 24" on center spacing?
Thanks,
Rich

Yes, a moisture barrier is highly recommended.

I would go no less then 16" centers. The old days they used 8" and 12" centers.