The problem is that ispconfig has it's own httpd service and runs as 'adminispconfig' user, not www/apache/nobody that most apache configs run as.
The other problem is that both the real apache and ispc apache want to bind to port 80 and they can't if the other gets it first.
You then have the option of either proxy forward depending on the ServerName and pass it to the real service (so real apache proxyforwards to ispconfig setup) or just use the port 443 port and begone with the port 80 idea.
The example below is a port 81 non secure with a proxy from the real apache server mapping an external name of (ispconfig.yourhostingdomain.com:80) to ispconfig (remember, 81 isn't SSL as it won't work).
# Port 81 is NON SSL of ispconfig
ProxyPass / http://localhost:81/
ProxyPassReverse / http://localhost:81/
CustomLog /var/log/httpd/ispc-access.log combined
Otherwise, 443 is just port 81 changed in the ispconfig httpd.conf