Code says only one vapor barrier is allowed on exterior tub and shower walls. The exterior walls and ceiling are covered with 4-mill poly. Both green sheetrock and wonder board are considered a vapor barrier.
Moisture does pass through the green sheetrock and wonder board, even though they are considered vapor barriers.
Once the moisture is on the back side of sheetrock or wonder board it then passes into the stud space.
In the winter the moisture freezes in the insulation. It melts in warmer weather, causing mold and rot. I’ve seen 2″ x 6″ studs rot away in 6 years.
Using a 40 Mill poly liner on the entire shower wall that is on an exterior wall stops all moisture from moving from the shower side of the shower wall stays on that side.
On tiled or stone/shower floors Use a 40 Mill poly liner on the floor and lap it up 12 minimums to 18″ up the sidewalls.