Another idea - the vapor barrier also needs to be placed on the *warm* side of the insulation. Otherwise, water vapor migrating from occupied spaces into the wall cavity may reach the dew point in the midst of the insulation. A problem if you are using fiberglass!
I also understand the concern about moisture originating from the block wall. A compromise: Place the vapor barrier on the warm side - just below the drywall. Then use foam board insulation against the block wall.
If moisture gets into the wall assembly, it won't hurt the rigid insulation. ideally, there should be a vapor barrier (and insulation!) on the exterior of the basement wall.