In a multiplayer game a lack of balance makes customization moot. There will be an optimal way of doing things that everyone does, and if someone finds a better way everyone will use that way.
I think the way to balance this is to not let players just copy things. If you capture a really good vehicle you need to take it apart piece by piece if you want to figure out exactly how to make it yourself. Then you need to be able to make the pieces. In real life ideas are frequently stolen but implemented differently, or wrong, and it fails to be better than the thing you spent time developing yourself. And if, for example, the metallurgy is beyond the thief's ability to duplicate, maybe he can copy the shape of the vehicle parts exactly but certain parts fail because they're put under too much stress. Also if he wants to create these vehicles he's going to have to retool factories to produce the parts. Meanwhile you're still pumping out these vehicles and his men are still having to fight against them. When his men capture one of your vehicles they can't keep it fighting very long because they don't have the right replacement parts, and possibly fuel / ammo / other consumables. By the time he has copied your design well enough and fielded large numbers of your vehicle, you've already developed a generation or two ahead.
In reality, it would be more like:
Player A: See I have a sweet gun I made.
Player B: Can I haz blueprints?
Player A: No, because I want to be the only one who has it.
Player B: *Kills him, takes gun*
Player B: *Tries to duplicate by hand, can't because he's a dumbass*
Player B: *Gives gun to friend who designs stuff*
Player C: I'll put the blueprints I made by reverse-engineering this on a forum.
Players D-T: Now I have an imperfect version of A's gun.
Players U-Z: We don't have anything because we're out of the loop.
Player A: That's OK because I was cool enough to develop the original gun, now I refined it into something a little better.
There's no reason to believe it would end up any worse than Minecraft, where people come up with cool designs for secret doors and stuff, and others can copy them. Except this game would have more complex designs, meaning a smaller population of players would be able to follow the steps.
Then you've got player investment in character skills. Will you spend your effort developing design and manufacture on your character or just fighting? You can always buy stuff from players so it's not like you need to design.