Go to OpenIV. Click tools and then go down to the manager bit (sorry if any of this is wrong, doing this from memory!). Make sure you all three of OpenIV tools installed, especially the one that enable your mods folder. Go to the dlcpack bit from your update.rpf folder, where it shows you all your patchdays, (make sure you are in edit mode before you make your way to the patchdays) and go to patchday13, copy it to your mods folder, there will be a message to automatically copy it, don't drag and drop it into the mods folder.
Now, when ever you want to add a new vehicle to your game, just drag and drop the vehicle files from wherever to the vehicle.rpf in patchday13 in your mods folder.
That should work.