The vendor class doesn't autoload
Both give me the vendor files. I have uploaded those files to the remote server's vendor directory, yet they are still not being loaded. 


What files did you upload exactly? It's not enough to upload just the new files from your package, installing packages with composer also updates its own autoloading files to make sure it knows where to load that and you probably forgot to upload those, that's the only explanation I have at least.

Try uploading all the files in vendor/composer as well or to be sure, all the files in vendor.