By virtual vfolder, I mean a folder that has no associated rule but has been
created as the parent of a "real vfolder". These are actually useful, they
appear when vfolders are arranged as the leaves of a hierarchy.

Create a vfolder named sourceforge/bugs. sourceforge appears as a virtual vfolder.
Move sourceforge to another virtual vfolder, trackers (to create it, create eg a
trackers/moodle vfolder). trackers could also be a real vfolder.
Try to change trackers/sourceforge/bugs's properties: an error message claims it
doesn't exist.
Go to tools/vfolder editor: it still exists as sourceforge/bugs.

Workaround: instead of moving sourceforge, rename sourceforge/bugs to
trackers/sourceforge/bugs.