This doesn't work for my particular case. I'm using a PHP framework which modifies 'session.save_path' at run time to store sessions in its own session folders (different folders depending on the user type).
Then you can e.g. remove the lines for the session save path from vhost template and php fpm pool template completely if you want to manage it on a per site basis. The templates exist so that a admin can taylor the system for its needs as we can not make all options available trough the interface as this would make the software unusable if we add a few ghundred additional fields with options in the site settings.