I wouldn't worry about vapor barrier, no shower I've ripped out has it behind the walls, but check with someone if there's intelligent life in the masonry/plumbing dept at your local store. (The Lowes I go to hires retired or out of work tradesmen for their depts, kinda cool)
I personally wouldn't use Durock, the stuff is a pain in the *** to work with. It's fine in whole sheets, but cutting it is a PITA. I'd use hardibacker, much easier to cut.
If there's anything more important than my ego around here I want it found and shot!