The vapour barrier must be installed on the warm side of the wall, that is on this interior side of the house. It is permissible to have it sandwiched in the middle between two layers of insulation, but only if the cold side insulating layer is at least two thirds thicker than the warm (source).
My question is what happens, exactly, if the barrier is actually on the colder side? e.g. two-thirds warm insulation, barrier, one third cold insulation? This is in a very cold climate (-40 in winter not unusual). And what can be done to mitigate this situation?