/etc/issue is far from being a reliable source for OS detection as it can be changed by a user without affecting any OS functionality. As newer Debian systems run systemd by default, check for /etc/os-release and fallback to lsb_release for older Debian versions. Check #7625 for a similar issue. Even lsb_release is not manatory, therefore keep the current code of parsing /etc/issue to avoid regressions.
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.