I'm building a new tiled shower stall with cement board backer. I've read that a vapor barrier should be placed over the wall studs before the board is installed, because the tile/cement board combination is not waterproof. We have an acrylic shower receptor, the directions for which also suggest a vapor barrier. I plan to use six mil plastic sheeting for the vapor barrier.

That's all fine, but what happens to vapor/water that get behind the cement board and stops at the vapor barrier? It sounds like I could develop a lake - probably not a good thing. How should the vapor barrier be installed so that moisture will drain/evaporate. (These are interior walls.)

Thanks in advance for the help!

Good question.

The Tile Council Handbook says a vapor barrier should be used when using cement board in wet areas (15lb roofing felt or 4 mil poly)

What you can do is set your shower pan first. What work well underneath that is "shower pan liner" sheet that is used in a mud bed application.
Set your pan first then place the vapor barrier on the walls and extend the barrier over the lip of the shower pan. Then install the cement board.

Great answer skiberger that is exactly what I do in all shower installs that use a pan liner!! That is why a lot of installers don't like to use a vapour barrier on the inside wall.if only they knew how important this barrier really is.structural damage to a home IS NOT CHEAP to fix!! make sure to overlap at least 2 to 3 feet and glue them.

Do not trim the bottom edge of your 6 mil until after you allow it to lay over/inside of the lip of the pan. The cement board and tile also hang over the lip so if water ever does get through the tile, grout and cement board it will drain onto the lip and into the shower. Your acrylic pan installation instructions should show this in a diagram. Also check if they recommend setting the pan in Structolite or plaster.