When in a vehicle (EG, the blackfoot), I can target vehicles using 'T'. However, the 'Next Target (In Vehicle)' button, R, doesn't seem to do anything. It simply deselects the targeted vehicle.
Even if I place three vehicles in front of the Blackfoot on the runway, I can target them by pointing the gunner at them and pressing 'T', but I can't switch targets using 'R'.