OK, I don't quite know how to do this..... but it seems to me that you may be able to extract the vehicles from the list of units in the trigger area by using the type command.
As an aside, I suggest you experiment with a known, named vehicle to figure out the best way to make it stop. That way, once we have figured out how to detect the vehicle, you'll know what to do to it.
Splitting the problem down into its constituent parts always helps.