Ah it's:
"%SystemRoot%\system32\vmwindow.exe"
...which presumably invokes the shell to open the folder (which opus incorrectly intercepts).
It also has a jumplist (useful) for launching the individual VMs - oddly these don't work (nothing happens) and nor does the stock jumplist entry "manage virtual machines". I suspect pulling opus off the system might cure this but I can't confirm that (yet).