I would imagine that depends on the design of the vent in the vehicle as much as it depends on the car mount. Personally - I wouldn't trust a vent mount for holding much more than a flip phone, I prefer the much more secure windshield mount. My iOttie OneTouch 4 mount has been holding fast to the windshield through all kinds of weather for 2 years now. Last time I tried removing it I practically needed a putty knife to peel the suction cup from the glass, so forget about it falling off randomly.
If you insist on using that mount, maybe some closed-cell foam stuffed between the louvers might keep them spaced and positioned appropriately?