I've added some lines to the "Apache directives" field, but I can't find them in the corresponding virtual host configuration file. Where are these directives being saved?
They get added in the virtual host file. If they are not there, then they are invalid and ISPConfig rolled back the last working config. You must have a vhost.err file in the Apache sites-available folder containing the directives and got rejected by Apache. You can e.g. name the .vhost file to .vhost.bak, rename .vhost.err to .vhost and restart apache to see the error messages on the shell.
