If you shoot a vehicle, normally the vehicle shouldn't blow up unless you hit the ammo rack or cause a spark in the fuel tank. If you shoot a vehicle with normal bullets, the car should be just disabled. If you hit the vehicle with any kind of explosive rounds such as 40mm HE round, 20mm HE cannon, ect. then it would be more understandable for the car to blow up. If you hit the vehicle with bullets, the car should just become unusable and have a smoke effect from the engine (if the engine was disabled) or just simply sit there.