Should I add a vapor barrier to kraft faced insulation?
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.