Vapor barrier goes on the warm side of the insulation (averaged year round)
If, on average, the garage is warmer than the house, vapor barrier goes on the garage side.
If, on average, it's cooler, it goes on the house side.
If you rarely close the windows, you probably don't need one at all.
Penetrations should be sealed with something like acoustic sealant (which never quite hardens, so it tends to last better than things that harden.) You can cut an extra patch of vapor barrier to aid in sealing penetrations by making a double-layer patch sealed to the main barrier.