re: Followup questions on wall repair

There is no house wrap on the building (just a pole barn). Right now, in that walled off section, from inside out it has OSB, paper backed insulation, then the plastic back insulation, and finally the metal panels. Not sure if I'll be able to find that plastic backed insulation for the small missing section where the AC unit was pulled out. Can I use some other type of insulation against the metal, and then try to slip in some plastic for a vapor barrier? 

(Really wish I could post some pictures to clarify!)