I have chosen vent mounts for some time as windshield mounts make my phone too far away and dash mounts have always left a mark after extended time with all the weather change.

The vent mount should be fine but yeas excessive heat will slow down the wireless charge to your phone. Cool batteries always work better than hot ones. When I turn on my heat though it comes from floor panels as heat rises. I use my vents for AC use mostly. Unless being used for side window defrosting.