Vapor Barrier Requirement in my garage ceiling

I moved in 17 years ago and I live up in the snow belt of Ontario Canada. The garage has been completely mudded and painted but I now have added a gas heater to warm it up a bit. The ceilings and walls are not insulated and when the drywall was installed on the ceiling there was no vapor barrier put in place for future insulating. I am wondering whether to cut the poly and staple in between trusses before placing the insulation. I just worry that the gap left under the trusses where drywalled will become a spot of moisture build up. Earlier posts state it may not be required however, my father believes I should do it