I had to re-install rFactor 2 because I accidentally deleted on of the default vehicles. After re-installing it that default vehicle was back. But now there are far fewer models in the \vehicles\ folder. Here's what was created. Here's a list of the only vehicles that were installed. Code: AC_427SC_1967 Brabham_1966 C6R_2009 Camaro_2012 Clio_PC ClioCup_2010 CorvettePC CorvettePCLights DallaraDW12_2014 FormulaRenault_35_2014 HondaCivicBTCC_2013 Howston_G4_1968 Howston_G6_1968 Kart cup_2014 Nissan_GT500_2013 SC2015_2015 USF2000_2016 Edit: Using the launcher menu, to uninstall a skin pack (used 1.94 as folder, instead of default 1.93), results in the enitre vehicle folder being deleted.
I think what you are seeing is the difference between the original ISI version and the cars added by S397 which was a rather extensive list of cars. But the list shown above is most likely ONLY the cars S397 has full rights or interest in. I suspect they are ignoring many of the old ISI cars due to lack of resources to further develop them.
I'm not 100% sure but that might be all of the default vehicles. Just subscribe to anything else you want from the workshop. You may have had some ISI vehicles that aren't on the S397 default install.
I went ahead an coincidentally restarted the game, and it went ahead and downloaded the files and such.