It is already included -- https://mediawiki-vagrant-image.wmflabs.org/mediawiki-vagrant/contents/Plugins/

and there are instructions in the README.txt file on installing it from the image:

6. Install the "vagrant-vbguest" Vagrant plugin:

    $ cd /USB_DRIVE_PATH/Plugins
    $ vagrant plugin install vagrant-vbguest-0.10.0.gem

Oh sorry. I see now that this is about the iso that is used to actually update the guest additions in the VM, not the Vagrant plugin that uses it. Is that iso not somehow bundled with VirtualBox?

The VirtualBox Guest Additions for all supported guest operating systems are provided as a single CD-ROM image file which is called VBoxGuestAdditions.iso. This image file is located in the installation directory of VirtualBox. To install the Guest Additions for a particular VM, you mount this ISO file in your VM as a virtual CD-ROM and install from there.

-- https://www.virtualbox.org/manual/ch04.html#idm1740

Yeah, I'm referring to the ISO.

Good point, I forgot it was bundled with VirtualBox. However, I believe that it doesn't actually use that ISO by default (it will still re-download it). It needs to be copied into the VM properly.

Probably need to add vagrant vbguest --do install --iso /path/to/VBoxGuestAdditions.iso to the steps (from https://github.com/dotless-de/vagrant-vbguest#running-as-a-command ). Or figure out why the local file autodetection is not working.