Hello,
Yeah the vehicule health GUI can't be modded directly so it will still show its original HP. However damage received will be reduced depending on the armor bonus from inserted platings.
You can test this with two vehicule, one with armor, one without and shooting them both a bit, you should see the armored one losing less health.
There's a gotcha however, if the vehicule takes more damage than its original HP in one shot, it may be killed still (exemple a car taking more than 450 damage in one hit).
This is because I can't prevent damage from being applied but health is "reimbursed" after the hit if possible (maybe it will be improved in the future).