Well it varies by game, generally in my experience at least whether or not the drivers are supposed to be shot, the drivers will be shot. But it really depends on the game, generally for ours they need to be hit with a solid round of some sort either tag, or a foam projectile. In either case the vehicle is down and then usually the guys inside pile out alive, though that varies depending on the field too. Sometimes I'll make vehicles not able to be engaged if they are part of a scenario say transportation, then allow the guys to get in shoot outs in the rear without being able to be killed or vice versus with drivers keeping the distance so it's not a complete hose fest. More in the case of a chase of sorts across the property. We did that with the deuce last year going to the next objective and I planned to have my dune buggy roll out after them giving chase with an M60 gunner on the passenger side. Added an interesting element to the game.
Generally I'd say your best projectile from a safety standpoint is nerf, but like anything it's ups and downs. ANd I honestly haven't used them myself, I've been around them and seen them in use allot. On my list is a grenade launcher and some nerf balls or rockets because we've started seeing them at games now. In the past they were just shower grenades mostly and other than being a cool shotgun pretty useless.
Luke