I have noticed that only about a third of vehicles destroyed - and I am only counting those for which you get the vehicle destroyed medal - count towards your commendations.
In order to get credit for your kills the following must be true
-If you board a tank/wraith you have to board it from the front and you must use nades.
-You must stay alive until they die, kills from the grave do not count.
-The vehicle must blow up on someone when they have enough shields. If they would have died had the vehicle not blown up then it does not count.
These requirements are a bit odd. In future Halo games the commendations regarding vehicles should either deal with the number of kills one gets with a given vehicle or only require the person going for the kill to kill the driver of a vehicle, not force them to blow it up under these bizarre sets of requirements.
I would have all of the vehicle commendations complete legitimately by now if it only required you to blow up the vehicles and get a vehicle destroyed medal.