Give this a try:
Do exactly this:
1. In OpenIV, go to:
update > x64 > dlcpacks> patchdayXng
where X = the highest number you can see. Note that it is not in the mods folder.
2. Right click on "dlc.rpf" in that folder and click "Copy to mods folder".
3. You should now be automatically inside the archive.
4. In that archive, go to
x64 > levels > gta5 > vehicles.rpf
5. Click the green plus button + and add the vehicles files from your download.
6. To make your life easier, right click on the file type bar (where it says Name, Type, Size, Attributes and Tags) and click on (no).
Good luck! If you have any other issues do not hesitate to reply back :)