Maybe take a few screen shots of the clips you're working on and the menu. Might show something that isn't obvious to you but is to others. This website makes screenshots REALLY easy:
http://snag.gy/ Check this folder too (or the appropriate folder where you installed SONAR if different)
C:\Program Files\Cakewalk\Shared DXi\VVocal
Make sure VVocal is actually installed!!! Run a plug-in scan at the folder that it is installed at if it ain't. Run the SONAR installer again and select ONLY VVocal to install as maybe it's not installed or corrupted itself.