If you go the plywood floor route I would lay pressure treated 2x4's on the flat and glue and powder nail (22 caliber nail gun) them to the slab and put rigid insulation in-between them. I would then use 3/4" plywood, tounge and grooved, and then screw and glue 1/4" plywood over it at a 90* angle and not put seams over seams and then fill any seams and gaps with a high performance floor leveler compound. This would prevent any seams from showing through or transmitting through the tiles. I wouldn't use any vapor barrier unless you have some pre-existing moisture problem, in which case that is a separate issue to address. My thinking is "either do it right, or don't do it". If you do half measures with plywood directly on the slab etc. then you will certainly run into problems later on and you would have been better off with just carpet. Generally, unless you have decent insulation or a really thick carpet pad, your feel may end up being cold or fatigued from spending mucho time on a slab.