Plastic is a bad vapor barrier..period. I would stick with the paper faced insulation and be done.
If you were to install plastic over the paper you would have 2 vapor barriers..can you see it raining in your home? This is a no-no in the vapor police book.
Paper lets the moisture collect a bit then releases it..plastic just runs down the wall. I have been in homes where the plywood at the exterior wall floor area was rotting through. These where extreme condition but better to er on the side of caution.