[herlichka]

That's a good question!
Here in Ontario, Canada, the building code accepts the combined qualities of your plywood sheathing, subfloor and finish floor as a "vapour barrier", as it occurs with an insulated floor over an unheated garage, or a cantilevered bay window. Given the fact that we have generous amounts of rain, snow and humidity and there is no specific requirement to add poly to the floor I would think that you would also not likely need one. I might suggest the installation of a housewrap like Typar or Tyvek. They form an air barrier to keep air from wandering into your insulation on a windy day, but allow moisture to dissipate, a handy consideration if you should ever have the misfortune of a toilet, tub or sink overflow, dishwasher leak or broken pipe.