Additional Vapor Barrier with Faced Insulation

The vapor barrier for a crawl space belongs above the floor joists and below the sub floor. You pretty much have three alternatives here.

1) Remove all the flooring - including the subfloor - install the vapor barrier - and replace the flooring, except you'll pretty much have to destroy the flooring in the process.

2) From underneath, take 6 mil plastic and set it into each space between the floor joists. You can then either wrap the plastic tightly around each floor joist, stapling it in place and then taping over the staples, or you can cut the plastic so that it doesn't wrap the floor joists, and tape each cut edge.

3) Buy spray foam and use it as both the vapor barrier and insulation. For this application you would want the fire rated open cavity foam like what they sell here: http://www.tigerfoam.com/products.php

Any high humidity area like a bathroom should have a good vapor barrier under the sheetrock. This barrier should be taped at the seams and ceiling forming sort of an air tight cell. Usually a kraft backed insulation is not a very good vapor barrier and has lots of gaps. To be on the safe side and avoid any condensation between the 6 mil and kraft, you could simply remove the kraft backing or cut several slices in it top to bottom. Usually it will not cause a problem unless you live in a very wet humid climate or the plastic is not sealed properly. The 6 mil is a much better material for your purpose than kraft backed.