A file is a Vfs when it describes a tree structure of files and directories. An example of Vfs is a zip file : a zip file contains files organized in directories.
Each Vfs file type needs a program in order to retrieve its content. For the ".zip" files, this program could be "unzip" or any other one which is able to understand this file format.
Scanning the content of vfs is longer because GTKtalog must call an external program. Once scanned (either in "Scan vfs" or "Full scan vfs" mode), a vfs file is shown as a normal directory. You can notice it's a vfs by the color of the icon (green by default). You can browse the subdirectories, the files of the vfs and interact with them in the same way as if they weren't in such a vfs (except about the unavailability of the file information). After a scan, the information field of a file inside a vfs is empty.
In the case of a nested vfs, the vfs inside the first one is not scanned as a vfs file (just stored as a normal file).
Right clicking on a vfs directory on the left frame of GTKtalog pops up a special vfs menu :
Edit vfs : Same as edit directory or edit file.
Vfs mode :
If the archive file was scanned in "Scan vfs" mode, you'll get the question : "Erase the virtual file system and only keep the archive file" ?.
Answering yes will delete the vfs and only keep the archive file as a standard file in your catalog.
Answering no won't do anything
If the archive file was scanned in "Full scan vfs" mode, you will get the question : "Convert this virtual file system and keep only the names".
Answering yes will convert the "Fulled scanned" vfs to a "Scanned vfs" (the file size and dates are dropped).
Answering no will delete the vfs and only keep the archive file as a standard file in your catalog.
Answering cancel won't do anything