Is that because of a lack of interest or just haven't done it?
If the latter, there are still ways you can be an effective team player and unlock stuff for vehicles.
Little Bird ... go engineer with a repair tool and be a passenger on the side, if you aim the repair tool towards the tail, I think, you can repair damage. Little Bird pilots can be very hard to bring down with a repair guy.
Tank Gunner ... a tank is more effective with a gunner, that's an extra guy looking around and spotting, shooting infantry with his gun, and hopping out to repair the tank when needed. Also, the gunner has specific unlocks which upgrade the tank if you are the gunner. Things like proximity scan which reveals infantry that's close to the tank, and the new incendiary which sets off an incendiary round around the tank to burn close-by enemy infantry.
Just for being in a vehicle, you'll get assist points for the kills that the driver gets.
I'll often drive a tank and you are welcome to hop in and gun.