Redoing the bathroom and replacing the bath tub enclosure with a tiled wall. The plan was Durock the bath tub walls and the waterproof with Redgard. But I have seen some people say you need to vapor barrier behind the Durock as well and others say it's not necessary if you seal the rock. Some say it depends on the board - Durock you don't need vapor, Hardibacker, you do need the vapor barrier. There doesn't seem to be a straight answer on it.
To add to it, one of the walls is an exterior wall and I've read you should vapor it anyway and others again saying no. Others suggesting it depends on the insulation - if it's faced insulation it already has a vapor barrier on it so plastic is not needed.