I've created a second user account on my Windows 8.1 computer to resolve an unrelated issue.
 
Everything about Sonar works for the new user account except for Melodyne. Do I need to re-install it? Is there some work around?

Is it having a problem accessing the Audio Cache? If so, it may need to be relocated where all users have access to the directory. The audio cache is set at the bottom of the Settings tab in Melodyne preferences.

The new user account has Admin Privs just as the old one did.
 
In Preferences > File >VST Settings I have
c:\program files (x86)\cakewalk\vstplugins
c:\program files\cakewalk\vstplugins
c:\program files\common files\vst3
 
I'm able to open Melodyne by launching the standalone program as the new user account.
 
I'm now Rescanning Failed and Existing.

And Whoop! there it is!
 
:-)