How to install insulation in a small space behind an existing shower?
Remove the shower and build the wall right
Or forget the vapor barrier and blow in cellulose.

If you really want a probable horror show, spray foam it, but I would not recommend that if you don't want a horror show - it will probably overexpand and warp or crack the shower enclosure.
If you don't insulate it that will be an uncomfortable shower (I've lived with that junk in places I didn't build. Would not recommend.)