ISPConfig is not setting /var/tmp as temporary folder. If this folder is used in your php.ini file, then it has been set by the php package maintainer of your Linux distribution and not by ispconfig.
The combination suphp + suexec does not make much sense fo php as suexec replaces suphp. suphp is very slow and should not be used anymore, the recommended php settings as described in the manual are php-fcgi + suexec or php-fpm + suexec. You will see that the upload temp folder is set by ispconfig then as well.