Is there any way to change the amount of modeled damage a vehicle incurs when hit by something?
Yes, somewhat, and no depending on what exactly you mean by "damage". I was never responsible for working on vehicles directly, so hopefully one of our vehicle guys will jump in here and expand or correct anything I talk about.
1. Hitpoints. All vehicles have a set number of hitpoints which control when they start to smoke, catch fire and ultimately explode. The base number of hitpoints for each vehicle is set in the vehicle table files, although the base hitpoints is sometime overridden at runtime (usually by script to give a vehicle more/less health depending on the gameplay requirements). Anything that causes damage to the vehicle will subtract from the vehicle's current hitpoints. You can adjust hitpoints in the vehicle table.
2. Components (doors, bumpers, etc.) can be unlatched or fall off completely. I'm not familiar with how exactly these various states are handled, and I think a lot of it might be hard coded.
3. Deformation. Certain pieces of vehicle collision can be flagged to allow visual deformation (dents/bumps/etc.). I believe the amount of damage is directly calculated from the velocities and mass of the colliding vehicles, but there may be some "toughness" parameters in the vehicle table somewhere that modify the amount of deformation. I'm not sure where geometry is flagged for deformation, but I imagine it's part of the vehicle's crunched data.
4."Crushing". Crushing is a specific type of damage added in SR3, to support - well - tanks crushing shit. Some non-tank vehicles are also flagged to crush things (the STAG SUV is the only one that comes to mind). This is specialized code, and does a lot of "smoke and mirrors" type stuff in the background to pull off. As such, tanks cannot be crushed themselves (two things crushing each other causes all sorts of glitches). There are crush specific values in each vehicle table to control how the vehicle is crushed "Crush_height" & "Crush_factor".
Like I said, I'm no where near an expert on our vehicle system, so there may be more tweakable values in the various vehicle tables to play around with. Hopefully some of these terms will clue you in for what you are looking for in the table files.