The VirtualBox provider can bootstrap to both .vdi and .vmdk images (raw images are also supported but do not run in VirtualBox). It’s advisable to always use vmdk images for interoperability (e.g. OVF files should support vdi files, but since they have no identifier URL not even VirtualBox itself can import them).
VirtualBox Guest Additions can be installed automatically if the ISO is provided in the manifest. VirtualBox Additions iso can be installed from main Debian repo by running: apt install virtualbox-guest-additions-iso
guest_additions: Specifies the path to the VirtualBox Guest Additions ISO, which, when specified, will be mounted and used to install the VirtualBox Guest Additions.
--- provider: name: virtualbox guest_additions: /usr/share/virtualbox/VBoxGuestAdditions.iso