After much research, I have a lot of conflicting advise about the need to add a vapor barrier before installing the backer (hardy / cement) board for my bathroom shower.
The "expert" at Home Depot, and another guy on youtube seem to think this is a very good idea, and even suggest sealing it with roofing tape. (The guy on Youtube tapes it directly to the sill of tub.)
However, when I asked about the tape and how far down I should go at Lowes, they told me they'd never hear of anyone doing that, and that it didn't seem like a good idea. Likewise, Ask This Old House only recommends putting down caulking on the sill before applying the backer board, and then again after before adding tile, and then caulking a 3rd time under the tiles before grouting.