A vapor barrier could be a paint-like product, but I would probably use whatever the flooring manufacturers recommend. This will keep you from voiding any warranty. If there is no warranty, I would use a plastic sheet, the heavier the better. There are other products available and I have seen plain old black builder's paper (tar paper) used. I like plastic because the laps do not stack up enough to show through the floor and if the liquid application fails, how would you ever know before pemanent damage was done?
The sub-floor does not have to be framed up. Sheets of OSB subfloor grade with a tongue and groove, fasten down with concrete fasteners like Tap-cons would be good. Tap-cons require a hole be drilled through the sub-floor into the slab, then the screw taps it's own threads right into the concrete with minimal surface damage. Hardened concrete nails cause the surface to bust and you lose that much contact/gripping area on your fastener.
Tx, InspectorD and I may have to do a road trip this year to see some of these projects. If we do, we'll have to come down there and see your stuff nd maybe you can go with us from there.
I want to be Moe, but I'm afraid I'm a Larry. Who will be Curly?Shemp?