V-Vocal is part of the X2 install, which is why you need X2 in order to have V-Vocal on your system, but once it is there, it is indeed accessible to projects in X3.
Lots of folks keep X2 installed for retaining access to V-Vocal in X3, even though you get the basic version of Melodyne with X3.
I don't know if they are still doing it or not, but when X3 first came out, once you installed it, you would get notification of a special upgrade price from the Melodyne folks, to get 50% knocked off the cost of upgrading to the top of the line version of Melodyne - which is WELL worth grabbing. The special price upgrade offer was good for 30 days, and if it is still in effect, I believe it was $150 to upgrade, rather than the normal $300 cost.
I hope I have cleared things up for you - X3 is my favorite release of Sonar since time began, and is quite stable AND has a lot to offer.
Bob Bone