Make sure Sonar is scanning the correct directory to find VVocal.
The .dll is called Cronus.dll, and by default is in C:ProgramFiles/Cakewalk/SharedDXi/VVocal
To access it , Right click on a clip and choose "Region FX - VVocal , or select a clip and choose VVocal from the Region X menu in the track view.
HTH.