The way the current damage system works for vehicles is extremely realistic and at times it seems almost random. The wrong parts of vehicles get damaged, and vehicle armor seems to be kind of all over the place. Take the new OpFor AH, with a 30mm cannon. Then put that up against an armored APC with a 40mm cannon, and the 30mm rips the APC up like a tin can, often disabling or destroying it with a single short burst of fire. Meanwhile, the AH can take sometimes dozens of 40mm rounds and fly out relatively unimpacted. But hit it in the side, and the 40mm explosive rounds wreck the rotors completely. Many aircraft are known to have taken direct hits from RPG rounds and did not suffer complete rotor failure, much less destruction. Even when the impact was close to the rotors (http://farm9.staticflickr.com/8091/8542455312_3afc72b797_c.jpg). I'm not saying this should be the result every time, but obviously these vehicles can put up with more abuse than they are currently capable of.
My biggest issue with vehicles right now however is what happens when they take critical levels of damage. Like... driving an ATV at just enough speed to take damage into a little fence and the thing explodes in a ball of fire and kills the driver. A simple collision should not cause a detonation in most cases especially with a vehicle like an ATV. A fuel truck, obviously a slightly different case (though a small, slow collision would not cause detonation immediately, or even soon or at all). In fact, generally speaking vehicles should not just explode from damage. Combustion requires a specific reaction. Even a bullet striking a gas filled tank won't cause ignition or an explosion very often, and if it does it's because it somehow causes a spark or source of heat that ignites explosive fumes. It'd probably take multiple rounds causing gasoline to leak and then form fumes, and THEN a round might strike and cause enough of a spark to cause a reaction.
I'm not saying that should all be simulated, but taken into consideration. Make a random chance based on how the damage occurred to start a chain reaction. Have an animation of leaking fuel to give a visual queue that the risk is greater, especially if an explosion occurs in the vicinity. Basic stuff. And collisions should almost never cause an explosion and not immediately. There should be a spread of fire. Have it start small under the hood or something, then grow through the vehicle to give a warning.
It'd be nice if the KIND of ammunition also had an impact on what happened if ammunition becomes a factor in the reaction. If it's just rounds, there shouldn't really be an explosion, just rounds cooking off. If it's something like a missile or RPG or explosives, obviously the effect would be different.
I'm just tired of the way especially set explosives and collisions impact vehicles. Most of the damage from an explosive at the ranges they do damage should be from the shockwave. I don't mind that knocking stuff out in the vehicle, but it shouldn't cause an explosion. Nor should most collisions. It's unrealistic, and if you want to have missions that involve support that allows repairing vehicles, or returning damaged vehicles to base, etc, it's kind of hard to do when any vehicle that is disabled is likely to be a burning heap of metal just because someone took a turn too quickly.
It'd also be nice if disabled/destroyed vehicles looked more realistic, instead of just black frames. Have a couple of models that kick off based on the place and even type of damage (or even just one for each type of damage: explosions, impacts, and bullets). Just to add some diversity and immersion to seeing a destroyed vehicle on the side of the road.