The beahviour you describe is absolutely OK, it means ISPConfig is working correctly.
Never edit Vhosts_ispconfig.conf manually as this file is always overriden by ISPConfig!
To figure out your httpd problem, please do exactly what i posted above:
1) rename Vhosts_ispconfig.conf to Vhosts_ispconfig.conf.bak
2) rename the latest Vhosts_ispconfig.conf file with a date appended to Vhosts_ispconfig.conf
and post the error message here.