Hi TrisBreezy, first of all sorry for my english, I'll try to do my best.
If you like to import a oac file with OpenIV 2.7 u still can. The only think is that the encrypt system was changed. To do it, first u need to export the vehicles.awc to openFormats (Right click above vehicles.awc - Export to openFormats (.aoc).
When you select a folder, it will decompress the vehicles.aoc and the folder with the sounds inside. You only need to overwrite your folder with the sounds, and import it like always. (The new vehicles.aoc will have the proper format and it'll allow you).
Now, if you don't have the vehicles.awc... then I don't know if you can decompress another one like movement.awc and change the name when is decompressed to vehicles.oac... I didn't try it.
Again, sorry for my english, if u need more info just tell me.