If you need to modify the max_input_vars directive for PMA, you need to modify the php.ini which belongs to the control panel (/var/www/fcgi/master/php5/php.ini) and once done, restart the imscp_panel service. For your single domain (assuming you're talking about a customer's domain), you must modify the php.ini file which belongs to it and depending on the httpd server implementation you're using, restart the php5-fpm service or the apache2 service.
I had modified the file (/var/www/fcgi/master/php5/php.ini) and had reloaded and restart the apache but not the imscp_panel service. I try thanks