edhe and rhirud hit the point best.
First of all, vehicles are certainly not overpowered in this game, all of them except the Lev are combatable on foot...particularly mantas, scorpians and hellbenders (as long as they don't have gunners).
This makes sense, they are vehicles, they are supposed to be stronger, and I think Epic got it spot on when it comes to their armour. I mean a tank...it does only take 4 Avril hits. Sure, going one on one with it and you are certainly going to lose sometimes....especially of the geography doesn't favour hiding places, but really...ITS A TANK FFS. IT'S SUPPOSED TO BE HARD!
The idea of vehicle damage affected the handling, in my opinion, sucks. That's something best saved for a realism racing game or whatever, not Unreal.
However, I don't think the idea of a spot on a vehicle which is slightly weaker and takes more damage is a bad idea at all. It would have to be as rare as headshots on people though.
Edhe's idea of the underbelly of a manta is nice as you usually only see that upclose when someone is trying to hop up and crush you...it would be nice to know that a very well placed shock beam could kill the manta just as he is about to pancake you.
On other vehicles I would guess a small place on the back...as most times when you are run over it is by someone going forwards, it would be nice to be able to dodge a scorp or bender and then hit it a few times on the rear "headbox" to take more damage off it than usual.
Tanks...hmmz...don't know as tanks move backwards as often as they do forwards. Perhaps a small spot on the back of the turret itself rather than the back of the main body. The turret is usually facing the action and it's therefore harder to hit the back of it. On the rear of the body though it would make a tank even easier for mantas to take out.