Vapor barriers are generally over-used. They make sense in severe cold climates like North Dakota, Upstate New York, Canada, etc. They stop the diffusion of water vapor from the warm, humid interior to the cold, dry exterior. At 40 below zero outside, with 10% relative humidity, there is a sizeable vapor gradient across the wall.
In Florida, or Houston, TX, they stop the diffusion of water vapor from the humid exterior to the drier, air conditioned interior. The vapor gradient is not nearly so great as the severe cold climates, however.
In moderate or maritime climates, vapor barriers make little sense. In winter time it is often more humid on the outside than the inside. There is no vapor pressure gradient trying to push water vapor through the wall. The vapor barrier gets in the way by preventing the wall assembly from drying to the interior, in the event of rain penetration, for example.
Air barriers are important in all climates. Leaking air will transport far more water vapor into a wall cavity than diffusion ever will. That's why drywall, plywood, etc should be applied and carefully taped and sealed to both sides of the studs.
For a garage-to-house wall, I'd use unfaced batts unless the garage routinely gets down to 10 degrees or less at night. I bet it rarely gets below freezing, unless you live somewhere really, really cold.
