Did you rename the original STS 2000P.dev to .dll? You shouldn't use that method - rename it back to .dev, and then go into you XTCApp folder. There should be a .dll in there that is named "VST Instrument" or something similar.
Copy THAT file into your VST plugins folder, and make sure the STS 2000P.dev file is in your XTCApp/Devices folder somewhere.
Rename the VST Instrument dll file to STS 2000P.dll, so when you scan for new plugins, it should pick it up.
I personally haven't used the Volkzampler, but that's mainly because the STS 2000P does what I need
