First off, forget the greenboard. Outside of the shower/tub area, regular drywall is fine. It is no longer in the national codes, but your area may not have updated. there are some newer fiberglass coated sheets (i.e., no paper which was the problem - green board still had paper, but it was treated) that work better, but are still probably overkill. Keep in mind, you really should use an exhaust fan or open the window (one or the other is required to be at least present in a bathroom, but doesn't dictate you have to use them!).
Plastic should go behind the cbu in a tub/shower area, lapping over the pan or tub flange. Roofing felt works as well, as well as a surface membrane (such as a paint on or sheet material). A vapor barrier is generally required in exterior or attic spaces and interior rooms, and again, plastic is fine for this.