Basement – Vapour barrier for basement garage ceiling

No, you only want a single vapor barrier. A second vapor barrier would create a moisture trap that would cause more mold issues. If there's a condensation issue, I'd replace the section of HVAC ducts with an insulated duct. The insulation inside the duct prevents the outside metal part of the duct from getting cold enough to result in condensation.

I'm in a temp range about 5C more in each direction. Attached garages tend to be about half as cold as outside so a thermal break is less important here. It wouldn't be a huge loss if you just screwed the gypsum board directly to the underside of the floor joists. However, it's easy to get a thermal break without much more effort, and it's almost as easy as applying directly to floor joists.

After applying the main insulation, sheath the bottom of the joists with 1/2" polyiso (polyisocyanurate) insulation board -- your thermal break. Then using longer screws, apply the gypsum directly to the joists through the polyiso.

Highest possible ceiling, thermal break if you want one, level application, no faffing about with hangers. Wins all around.