Vapor Barrier Advice
It's highly likely you don't need or want an actual vapor barrier here. These horrible things are still popular in the frigid north climes like Canada, but they truly aren't needed in a well-insulated wall like the one you have. Even up there, on average they cause more problems than they solve and substantially reduce a wall's ability to dry. They can also become mold machines if the space is ever air conditioned and you have stucco or stone wall cladding on the outside. Good choice using mineral wool.

Bottom line: skip the vapor barrier and don't worry about it. Just install your drywall.