Yesterday, we tried the Cubase Cross Platform Converter.
And indeed, after converting the Cubase project file from Mac to Win, the Virus Control VST3 plugin was properly loaded.
Naturally, I was curious what the Converter actually changed in the cpr file. A binary diff revealed that a value which looked like an ID or GUID was changed throughout the document. When doing a conversion back from Win to Mac, the value was changed back. So I wondered what this could be, and a Google search for "vst3 plugin ID" dug up a couple of interesting bits. One of them pretty much explaining the basic problem of the situation:
VST3 Plug-In ID Changes on Mac
So, the VST3 version of the VC plugin for Mac and Win use a different GUID. Which is an incredibly unfortunate mistake that could be fixed in the plugin, but would require either all Mac users or all Win users to convert all their projects.