The 'wall',as in stud bay, can not rot with the vapor barrier in place.
Green board or moisture resistant drywall is just that 'moisture resistant'. Meaning that moisture can pass through it easily enough and will travel to the vapor barrier in a 'longer time', than with just regular drywall.
If this theory of a second vapor barrier were true, then you would not need a vapor barrier under regular drywall either,'theoretically'.
What you have are people who only read and do not work for a living. They see the drywall says moisture resistant and think it's a vapor barrier. the real question is "is the resistancy of the green board , equivalent to a 2 mil or 4 mil or 6 mil vapor barrier. Or is the green board only equivalent to a 1 mil vapor barrier. This will result in a 'below code' vapor barrier on the wall. Meaning you will need a further 3 mill vapor barrier to make code.
This is a good number to try and google in your search. Or call/email the drywall manufacturer and ask them what 'vapor barrier mil thickness' their green board is..There is doubt they even know. but if they have the number, you have overthrown the building code error that exists.