I think the explanation has to do with the language used for Windows.
There is nothing special about the name "Program Files", it just happens to be what the standard Windows program folder is called in English versions of Windows. If your native language is something else, Sonic Visualiser and other Vamp hosts will look in whatever the programs folder is called on your system.
In the case of German, I believe that means you should create your Vamp Plugins folder inside C:\Programme. Try that and, if it works, let me know and I'll update the documentation to mention it.