ok then make a copy of your /etc/apache2/vhosts/Vhosts_ispconfig.conf.
cp /etc/apache2/vhosts/Vhosts_ispconfig.conf /etc/apache2/vhosts/Vhosts_ispconfig-backup.conf
Now remove all the php_admin entries within the original Vhosts_ispconfig.conf
Then restart apache
Then you should be able to finish your suphp install.
You will probably not have to do any edits to your php.ini then looks as if their is possibly a step missing in the howto.