Should I use vapor barrier for a framed floor over a slab?
The best place is for the vapor barrier to be is UNDER the concrete slab.  This prevents moisture from the ground from seeping through the slab into the living space.
Assuming that's not the case, then your next best choice is to apply a moisture channeling product on top of the slab.  You don't want to use poly film here as that will cause moisture to condense between the slab and the poly with nowhere to go.
There are any number of commercial moisture channeling products but these provide both a vapor barrier and a system of channels for the trapped moisture to drain, most often to your sump pit.
Even though your floor is &quot;dry&quot; now, that doesn't mean there is no moisture coming through it.  Concrete is porous and allows water to penetrate.  The reason it seems dry is that the moisture is evaporating into the air.  Try taping a sheet of poly to a small area of the floor for a few days and you should see moisture trapped under it.