I read the thread and got it working, even though I did the first part last. My fault on that one.
1) You need to do the bin file copy and ServerUnlock file download and upload thing or you will not see the cars in the screen on the dedicated server as you walk through the screens even if you have them installed with the Modmanger. Please read the first page of this thread. It's easy. What this does is that it creates a file on your local computer and finds out who you are. That way, when the file is uploaded to your server it knows the content belongs to you and lets the car show up.
2) I was lucky that a post in the thread showed the IDs of the exact cars I wanted to use, BUT, I didn't use the CMD file update method. If you get content from the Steam Store, the ID is not listed in the URL, like stuff from the workshop. I used the following steps, as mentioned by another poster earlier in the thread.
3) After you have purchased and synch'd to the steam workshop and the content is installed on your computer, you need to copy the specific vehicle folder from your /rfactor 2/Installed/Vehicles folder. Get the whole folder for the car and upload it to the same folder on your dedicated server.
4) When you are making a "VMOD" or an Event you also need to ONLY pick one version of the vehicle, if there are more then one version in that folder. If you choose both, the package creation will have an error about having two files of the same name. It can't create 2 rFm files with the same name.
I may try the CMD update version next time, but I need to figure out how to learn the ID of the Store paid for content for that to work. Maybe there is a log file ?