Wipe computer and start over. When you try to install an earlier version any library files which are the same name, but with a later version stamp, are likely to be left in place by the installation program and not overwritten. This is a very common problem when trying to downgrade for any software package. This leaves you with commingled versions of software whose behavior is unpredictable and almost impossible to support if you do have an issue.
If you need a particular component, such as VUGEN from an earlier release, then consider running a virtual machine on your PC and inside of that VM run the version of VUGEN that you need.