@naterezi
Been a while since I made similar edits to my own game but I will give it a go. When I did do it, these were my findings: 
STARTING VALUES:
In 'vehicles.meta' you have:
<defaultBodyHealth value="1000.000000" /> (raise it to whatever you like to give your vehicle more HP to soak up bullets & damage before it dies on you)
& in 'handling.meta' you have:
<fWeaponDamageMult value="1.000000" /> (lower this value to make the vehicle tougher/take less damage from weapons. This is the more influential of the 2 values)
Try editing those two values to something like this:
<defaultBodyHealth value="20000.000" />
<fWeaponDamageMult value="0.001000" />
& see if it toughens up your vehicle/s enough (I suspect it will do the job for bullets but explosive weapons will still destroy the vehicle with one hit no matter how high/low you set the values. Don't worry, if that happens, I have a solution. See 'VEHICLE STILL DESTROYED BY SINGLE ROCKET:' below ).
Feel free to play around with the two values above to the extreme (2000000 or 0.0000001 etc) & test the results as you may well find something I have missed or didn't work for me etc.
As you'll see in the 'Damage Output Observations:' & 'Non-transferable Settings:' sections below I found a wide range of differing results on a vehicle per vehicle basis just from editing the three values mentioned in this post. Experimenting with values to find the perfect settings for your game will be key
Note: Remember to make backups before editing anything I favour backing up the entire '.rpf' archive/s if there's enough space.
Background:
I've edited my game so that it takes more than one rpg/rocket etc to destroy the tougher armoured helicopters ('Savage', 'Hunter' etc) & although I have the effect working 100% correctly 
I'm not sure all the changes I made to the helicopter's meta files are needed for it to work?! (being lazy, I haven't gone back to check). I spoke to other modders about it & there seems to be some debate as to the minimum needed to get it to work. I've only tried it on Helicopters so maybe land vehicles are different somehow/other variables etc. Hopefully, if you can confirm if the above edits allow a vehicle to survive a RPG hit (they don't for me last I checked) I can find out if I am alone in that my game does this or if it works for others too
VEHICLE STILL DESTROYED BY SINGLE ROCKET:
Leave the values something like this (so the vehicle is very likely to survive more than one hit once you make the '<strHandlingFlags>' edit below):
<defaultBodyHealth value="20000.000000" />
<fWeaponDamageMult value=="0.001000" />
& then in the vehicle's 'handling.meta' file find the '<strHandlingFlags>' value:
Example:
<strHandlingFlags>C400100</strHandlingFlags>
Note: The values in your line will be different depending on what vehicle you are testing.
The key part is to enter a 'C' in the 7th digit position (ie 7th digit from right to left). This handling.meta tutorial by @V4D3R contains a brief description & a chart showing the effect of the digits 1>8. It looks like this:
Example (correct digit positioning):
<strHandlingFlags>87654321</strHandlingFlags> (max of 8 digits possible)
Example (7th Digit Replace):
If there is already a digit in the 7th position then replace it with a 'C'.
<strHandlingFlags>8C654321</strHandlingFlags>
Example (less than 7 digits total):
If there are less than 7 digits in the line, replace the missing digits with zeros ('0') to get to the 7th digit.
Change this:
<strHandlingFlags>4321</strHandlingFlags>
to this:
<strHandlingFlags>C004321</strHandlingFlags>
Note: I have no idea why this 'C' value change works. I just saw it in the 'Rhino' & Freight Train's handling & decided to test it out. Given I had nothing else to go on it worked out quite well.
Well done for getting this far by the way At this point your vehicle should be fugging indestructible!
FINDING THE CORRECT VALUE/S:
Your next step is to edit the '<defaultBodyHealth value' (lower it) AND/OR '<fWeaponDamageMult value' (raise it) until you are able to destroy the vehicle in the correct number of rocket etc hits you want.
Try somewhere near these values (no promises. See 'Non-transferable Settings:' below):
<defaultBodyHealth value="1500.000000" />
<fWeaponDamageMult value="0.010000" />
The '<fWeaponDamageMult value' is the most influential value of the two above. Test edit it first & then play with the '<defaultBodyHealth value' once you get a feel for what is going on (or lower the health if you think you may have set it too high etc). Finding the sweet spot may take some time but stick at it. It is possible
If you are finding it hard to find the right settings either jump your values from one extreme (indestructable) to the opposite (1 hit kill) as you test them out. Slowly making the changes less extreme (ie closer to each other in value) & you will naturally gravitate towards the 1 hit/2 hit etc crossover point.
Or alternatively, try the 'ah ha, it blows up after 78 rockets!' approach & then raise the '<fWeaponDamageMult value' incrementally to reduce the number of rockets needed until you get to where you want
Non-transferable Settings:
One thing I have noticed, is that much like the vehicle handling values in GTAV, the settings you input for a certain vehicle don't transfer over exactly to a different vehicle.
It's like each vehicle has a different starting health/toughness value (may be other variables I am yet to discover at play here too) meaning you will have to tweak each vehicle separately to control (for example) how many rockets it takes to destroy it.
Example:
Two cars are set identically with these values:
<defaultBodyHealth value="2000.000000" />
<fWeaponDamageMult value="0.030000" />
Car 1 blows up with 1 hit but Car 2 blows up with 2 hits?!
Solution:
Raise Car 1's '<defaultBodyHealth value' AND/OR lower Car 1's '<fWeaponDamageMult value' (more likely to work for rocket/explosives) until it can survive a rocket blast ALL (see 'Damage Output Observations:' below) of the time
Damage Output Observations:
The second thing I have noticed is that there is a lot of variation in the damage output
- of any given explosion (probably to do with the games physics calculations)
- & where exactly you hit the vehicle (Tip: Rhino is ~2x more vulnerable if you hit rockets underneath it )
This means that during testing it's a good idea to fire a LOT of rockets at a LOT of different parts of the test vehicle until you are 100% certain it survives (for example) EVERY first hit no matter what & is destroyed by the second hit EVERY time too
That should keep you going anyway. I think I covered the basics lol (was meant to be just a quick answer It's now dark outside lol).
Do let me know how you get on/if you have any issues/questions etc
Best of luck
