Ok, I know ya'll aren't keen on the HP system, but hear me out. This can easily be used to our advantage, and goes the same way as the body armor simulation I proposed elsewhere.
Divide vehicle hitbox from fire geometry. Place fire geometry to be hit -inside- the vehicle and assign armor values to external hitbox (ie, the vehicle as seen from the outside.) using rvmats.
When firing at the vehicle, the shell needs to penetrate the external hitbox. Inside the vehicle, roughly shaped fire geometry for the fighting compartement and all damageable components are placed.
If the attackers shell does not penetrate the external armor shell, nothing happens.
If it does penetrate, it strikes the internal fire geometry and does according damage to the vehicles HP. No change to the HP system. I detail the advantages I see immediately below. I will also draw a picture to illustrate my Idea and add it to this post.