First off, I am remodeling a bathroom in a 33 year old home in Phoenix AZ. The bathroom has a 3'x5' shower stall with a standard sink and toilet. I have ripped all the drywall out and the tile. The tile is original and was adhered to the drywall. Surprisingly ( or not) there was no mold behind the drywall. Two walls of the shower are exterior walls and the third is an interior wall. I have gone round and round in my head about what backing material to use for the new tile wall to be put up in the shower. As it is, the old drywall held up for 33 years. But from my understanding, drywall is the old way of doing things. I think I have pretty much convinced myself to use Durock as the backer board. For the exterior walls, where I will be putting Durock, is a vapor barrier really necessary (6 mil poly or felt)? If I water proof the Durock do I need the vapor barrier? The exterior walls will have new faced insulation installed. When I butt the new drywall up to the Durock, how do I join the seams? Do I use standard drywall mud and tape or is there a different technique?
Any advice will be greatly appreciated.

Dan