Vagrant has been failing to run on CentOS 6 since version 1.2.5. The read of file /etc/redhat-release appeared to be only checking for the word Fedora. Altered the regex to match CentOS as well
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.