Flooring Tile - ceramic tile as trim

Ok, My new idea for solving the uneven top side of new cultured marble surround is to use bullnose ceramic tile (matching the floor) as a border above the surround (3 sides) and then down to the floor outside of tub. The grout filler will then be adjusted to allow for the area that is unfortunately 1/4" too tall. Surround is 60" high and is properly installed on durock. However, for the 2 or so feet above the surround, there is only drywall (maybe greenboard-can drywall be green? Sorry to be dumb.). This area does not get wet, but will get humid. Can I still install the bullnose onto the drywall? Also heard of durock's new vapor block. Can that be used under the tile in this application, or am I totally goofed up? I guess the drywall can also be torn out and replaced with durock, for those few inches, but it would be nice to not have to make a mess again if drywall is ok for this situation. Thanks! Trying to become educated. Sharon