There isn't much point to a loosely stapled vapor barrier, since it's also serving the function of an air barrier that allows to the fiber insulate to operate at spec. If the polyethylene vapor barrier has gaps top/bottom/sides or major holes in it it's not even that great a vapor barrier, since it's allowing convection forces to move air from one side to the other. Small air leaks can move MASSIVELY more moisture through a wall than vapor diffusion alone.
Best practices would be to AIR SEAL THE SHEATHING on the exterior prior to insulating, and install a tight fitting "smart" vapor retarder such as 2-mil nylon (eg Certainteed MemBrain) on the interior side of the studs & cement board, detailed as an air barrier as
per the instructions. Asphalted kraft facers are also smart vapor retarders, but can never be made truly air tight.
To air seal the sheathing (probably OSB or plywood under your exterior foam?) seal the full perimeter of each stud bay to the sheathing using polyurethane caulk (or purpose made products like ProPink, etc.), and seal any seams in the sheathing with housewrap tape, reinforced with a 1/8" layer of duct mastic to 1/2" or so beyond the tape edges to ensure good adhesion over time. Be sure to put a bead of caulk between any doubled-up framing such as top plates, jack studs, etc and where the bottom plate meets the subfloor.
If there is no structural sheathing, use foam board construction adhesive to seal the foam board to the framing on all edges of every stud bay, and reinforce any seams between framing using housewrap tape + duct mastic.
Unless the exterior foam has a foil or vinyl facer it's not a true vapor barrier until it's at least 1.5" thick (for XPS- pink, blue, green, sometimes gray) to even hit class-II vapor retardency (=< 1 US perm). Six mil polyethylene runs about -0.05 perms, foil facers even lower still, making them true vapor
barriers (= <0.1 US perms). Anything under 5 perms on the exterior does a pretty good job of protecting the structural wood from the severe moisture drives of sun on damp brick, while still allowing some drying capacity toward the exterior. But installing 6 mil polyethylene on the interior traps that moisture in the studwall, with
no drying toward the interior.
In a TN climate in an air conditioned house this can lead to condensation running down the exterior side of the polyethylene whenever the outdoor dew point is above the indoor temperature even without the moisture drive of the brick, but with a brick clad structure it's pretty important to preserve some drying capacity toward the interior, ergo the prescriptive for using smart vapor retarders. When the air entrained in the insulation or behind the cement board is fairly dry (<40% RH) the smart vapor retarder runs about 1 perm or less, but when the moisture levels are high it become vapor open, allowing moisture to pass relatively freely, preventing moisture from becoming trapped in the stud bay.
Yes,with smart vapor retarders there will be some moisture transfer from the interior side into the wall assemblies after showers or when the bathroom humidity is high enough for condensation on the walls & mirrors, but those events are transient, limited to a few hours per day- assuming this is a house and not a public bath house. As long as the vapor retarder is AIR tight the amount of moisture getting in to the walls via diffusion is minimal.