With https://community.bistudio.com/wiki/addVehicle one can make vehicles available for AI groups to use, as well as to mark them, even empty, as threat to the enemy.
With https://community.bistudio.com/wiki/leaveVehicle one can unassign vehicles again - but also makes the units disembark.
For full control and more flexibility, it would be useful to be able to unassign a specific vehicle or all vehicles (without impacting the AI units in the vehicle).
unassignVehicles group vehicle unassignVehicles group [vehicleOne, vehicleTwo] unassignVehicles group