Which part of Australia do you come from?

I don't come from Australia, but I had a wonderful vacation there not long ago, so that must have come through.

In my case, that device was an external T: drive, which was already connected at startup. As a test, I shut down, disconnected T: then repeated the test. Now the drive is not available (I think it was marked with a red cross in the Macrium UI.
If sounds like the UI you were looking at was the Existing Backups tab and the red X was on an entry in the "Folders to search" list that is used for populating the Existing Backups pane. The better place to check is the Create Backups > Local Disks tab, which shows all storage devices that are currently visible. One reason for that is that Windows and Rescue do not always assign drive letters the same way, even though Rescue Media Builder does copy some data from Windows into Rescue Media to improve the likelihood of consistency. But in the case of a drive letter discrepancy, it's possible to have a disk online and available within Rescue and still have a red X on that path because the disk is currently using a different drive letter. In that case, you can use the Local Disks tab to check the currently assigned letter, then click the Edit button under Existing Backups to add the new path to the list. Changes like that made in Rescue do not persist, fyi.
Rescue can also work with devices that are connected after it loads, but for storage devices, you'll want to click the "Refresh" link in the upper area of the Create Backups > Local Disks tab to have Reflect rescan for storage devices.
In any case, if all of your backups are on local storage, then as long as Rescue can see all of your local storage, then you're covered. Or "happy days", to borrow further from the Aussies.

But I'm wondering when I would need to use a bootable external drive if I already have the Macrium vboot procedure installed.
This confuses me a bit. There's a feature called viBoot (not vboot), but that allows you to boot backups as virtual machines within Windows, which has nothing to do with restoring in Rescue Media. If on the other hand you were referring to the recovery boot menu option whereby you can boot into Rescue right off your internal drive rather than an external drive, then there are many scenarios where that will not be possible. The files on your internal drive that facilitate that may have been deleted. Or file system corruption might have rendered those files inaccessible. Or ransomware might have encrypted the entire drive. Or your drive may have simply failed, requiring you to install a new one that will be blank. In all of those cases, the files to boot into Rescue from internal storage will not be available to you when you will likely want to perform a restore. That is why I advise people that the recovery boot menu option should be viewed as a convenience, and while it can even be your primary means of performing restores, it should never be used as your SOLE means of doing so. By comparison, external Rescue Media has no such dependencies on any files existing on internal storage, and therefore it CAN be used as a sole means of recovery if you don't want to deal with the boot menu option -- but even if you do, having external Rescue Media should still be considered a requirement to cover more significant incidents.