httpd service will not start on boot under CentOS 6

The problem turned out to be that the VM was trying to start the Apache service before Vagrant had mounted the shared folders, and the httpd DocumentRoot was pointing to a non existent directory. I followed these instructions to set up a udev event to wait until the mount occurred before attempting to start httpd.

http://razius.com/articles/launching-services-after-vagrant-mount/