do I need to put my vinyl membrane behind the backer board in a shower stall?

Answer 1

It certainly can't hurt; but I suggest you could tear out a thousand bathroom shower stalls and not find a vapor barrier in any. More directly however if you're using a vapor barrier it should be attached to the studs, not the face of the backer board.

Steven Wolf

Answer 2

If it's the shower, the membrane should run from the pan up the wall about three feet behind the backerboard. This makes sure that the water doesn't get between the cracks and rot the wall.