The version_number regex was causing an error because my /etc/redhat-release looks like this: Fedora Core release 6 (Zod) This fix allowed initialize to run just fine on my ridiculously old release.
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.