Insulation, Radiant and Vapor Barriers - Insulating Cathedral Ceiling walls in attic

I am in the process of purchasing a home and know of few things that need to be done and one is to re-insulate the cathedral ceiling walls in attic. The paper backed fiberglass insulation is on the wrong way and is also falling down and there is no vapor barrier. The cathedral ceiling is in a bathroom. So I what would be the best way to fix this?

My thoughts were to either use rigid foam, just not sure on how to seal it to have a vapor barrier, or just use spray foam if its not to expensive. Maybe a 300 sq ft total.

Any other suggestions?

Thanks

This link has a good description of vapor barriers, although the examples shown relate more to exterior walls.
BSD-106: Understanding Vapor Barriers — (http://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers/?full_view=1)

Bud