It seems that the proper way to do this, in fact the only way that could find, was to download VDF-9.1 from the ftp site, install it and run "Register Workspace". Without doing that, VDF-11.1 can't find the application to convert. In my case I was given the source in a zip, missing several files to boot, and have never had those old versions on this box.

Once registered in 9.1, the 11.1 Import wizard was able to see it and make the conversion to 11. And then converting to 18.2 worked as expected.

Now to see what else needs fixing -- menus, missing files, etc.