Hi Brian, welcome to the forum.
First, does the volume issue happen when VDL is in stand alone mode? If yes, then...
Inside the directory where you installed VDL there is an "Instruments" folder (see attached image). Delete everything inside, then copy/paste the same contents back into it from the VDL 2.5.5 installer files. If that doesn't take care if it you may need to do the same with the .nkc and big .nkx files - or, just go ahead and do a total reinstall. And if that still doesn't work, you'll want to get with the Tapspace guys so they can help you get it sorted out.