Here is the official Microsoft documentation for the version number hack jules mentioned:
https://msdn.microsoft.com/en-us/library/windows/desktop/ms724451
Basically, if you want the Windows version to be reported correctly on Win8.1 and Win10, you need to put an extra thing in the app manifest saying that your app supports Win8.1 and Win10.
However an audio plug-in cannot do that as it is merely a DLL loaded in a host. So to fix this, the host (Reaper in your case) needs to have that app manifest thingy added. Nothing we can do about that!