To use these cars copy them into your I76/Addon directory.
Then copy them over (rename them) to files in the directory; 
This replaces one of your cars variants with the specified vehicle.

For instance.. I have the file "vcmanta4.vcf" in my Addon/, this is
my first Manta variant. If I wanted to use the tank I would copy tank.vcf
over vcmanta4.vcf and check the Manta's 4th variant for a tank.

If you don't want to overwrite your variants create new, dummy ones and 
overwrite them.

Also, after replacing a variant file (.vcf) you need to restart I76 in order for
the program to reread the files. 

--
crymson