Enable virtualbox-* builders to preserve .vdi format when chaining templates so disks can be resized in later stages about packer-plugin-virtualbox HOT 2 OPEN

you could go with specifying import_opts = "importtovdi" in your source, but that still leaves you with the problem that vboxmanage modifymedium expects you to know either the full path or the UUID of the created disk.
You might be able to guess/hardcode the full path though

from packer-plugin-virtualbox.

Did the suggestion from the forum work?

https://discuss.hashicorp.com/t/enable-virtualbox-builders-to-preserve-vdi-format-when-chaining-templates-so-disks-can-be-resized-in-later-stages/23815/2

Off the top of my head I think you want to use the keep_registered flag in the virtualbox-iso build and then use the virtualbox-vm builder rather than the virtualbox-ovf builder. You don’t even need to pass in the .vdi file, just reference the name given to the vm in the prior build by setting vm_name.

from packer-plugin-virtualbox.