Well, unless required by your local codes, the WR wallboard is no longer part of the national codes...
Most locales require a shower to be built to the guidelines in the Tile Council of America (TCA) handbook. Short answer...yes, put a vapor barrier behind the cbu on the walls of the shower and run it down over the top of the flange on the pan. If the other walls are not an exterior wall, no problem. Make sure to follow the manufacturer's directions on the number of screws and to install the alkalie resistant reinforcing tape to the seams. You can do that while tiling to prevent 'speed bumps'. It isn't like mudding a drywall seam - it isn't to look pretty, but to reinforce the seam.
Unless prohibited by the manufacturer (unusual), you will get a lot more life and enjoy it better if you bed that pan in mortar to fully support it since most of those things a flimsy when walking around on them.
A really good place to ask tiling questions is www.johnbridge.com.