VirtualBox has a bug where the IPv6 route is lost on every other configuration of a host-only network. This is also triggered when a VM is booted. To fix this, we test the route-ability of all IPv6 networks, and reconfigure if necessary. This is very fast but we still only do this if we have any IPv6 networks.
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.