Please review /etc/httpd/conf.d/viewvc-fcgi.conf and /etc/httpd/conf.d/viewvc-wsgi.conf
Do you have any error output or some other piece of info to show us what actually is wrong with the config? I don't have an F-18 installation handy to check (i.e. everything was just inherited form previous packages).
Look at bug #871373 (blocker of this one) which provides some explanation / example.
viewvc-1.1.17-2.fc18 has been submitted as an update for Fedora 18. https://admin.fedoraproject.org/updates/FEDORA-2012-16646/viewvc-1.1.17-2.fc18
Package viewvc-1.1.17-2.fc18: * should fix your issue, * was pushed to the Fedora 18 testing repository, * should be available at your local mirror within two days. Update it with: # su -c 'yum update --enablerepo=updates-testing viewvc-1.1.17-2.fc18' as soon as you are able to. Please go to the following url: https://admin.fedoraproject.org/updates/FEDORA-2012-16646/viewvc-1.1.17-2.fc18 then log in and leave karma (feedback).
viewvc-1.1.17-2.fc18 has been pushed to the Fedora 18 stable repository. If problems still persist, please make note of it in this bug report.