The last version of SONAR to bundle the Cakewalk version of V-Vocal was X2. You will need to perform an advanced install of an older version of SONAR. It will take a minimal installation of the old version to get V-Vocal back.

I would recommend finding an alternative to the plug-in soon. The plug-in is no longer maintained. Even when the plug-in was in active use, it seemed difficult to get Roland to fix the problems with the plug-in. This was a Cakewalk plug-in in name only.