I imagine this should have been done by the ISPConfig installer.
Do a quick check to see if the include exists using:
Code:
grep -iR Vhosts_ispconfig.conf /etc/httpd/
If that doesn't bring anything up, stick this line at the end of your httpd.conf:
Code:
Include /etc/httpd/vhosts/Vhosts_ispconfig.conf
Hope that works