How can I enable vehicle indestructibility and where exactly can I find the serverconfig file?

This is not changeable.

I read the FAQ but can't find the serverconfig file exactly where it is ?

You cannot add an option in the configuration file so that the vehicle does not receive damage when it collides with another vehicle ?

There are two types of configs. The config for the server side is located in your world folder in a subfolder called 'serverconfig'. The client side config is located in your '.minecraft' folder inside a subfolder called 'config'. Note that the client side and the server side config isn't the same. You can't put stuff from the server config into the client config. If you can't find the option you are looking for in the client config, try looking into the server config. There is always a server config, even if you are playing singleplayer, as it also uses an internal server.

Next time find it yourself!

I have seen that before !!! but i can't see this file in the world folder

I used modifications on one of the previously created worlds, I wanted to view the file but in the saves folder there is no file or serverconfig folder.

???

What do you mean with indestructibility? Read the FAQ.

that the vehicle does not suffer any damage when hitting a block