Yeah, being able to hit things you can't see is one of the things that really lets spells like Fireball shine. Just need LOS to the target point, and FWOOSH! "Oh look, there's someone leaning out of that helicopter with a machine gun." Fireball inside the helicopter. "Well, the person with the machine gun is NOT happy with you. He's even less happy with the pilot who is giving off this really smoky smell, and is leaning forward on the controls like he's dead. Oh, that's because he is. Now for the happy fun crash!"
Remember, you don't have to kill the vehicle to stop it, just kill the guy driving it.