Vapor barrier on Bathroom tub exterior wall(s)?
The liquid membrane acts as the vapor barrier so you don't need plastic behind the cement board.  Redgard data sheet says 0.35 perms at 30 mils so you can get away without an additional vapor barrier if you put on enough of the redgard.  Do the same for the interior walls as well.  Use a good quality primer and paint for the ceiling above if that gets drywall.  Both Benjamin Moore and Sherwin-Williams make vapor barrier primers you can use.
When looking into this before, I went with Mapei AquaDefense which is a similar product but with better specs and a less &quot;redrum&quot; appearance.  I think RedGard may have updated their specs since then.  Leave a narrow gap between the cement board and tub, make sure to paint the redgard over the bottom edge of the cement board, and caulk the gap.