The first Kingdom Hearts, which is essentialy a Spinoff game to Final Fantasy sold 5 mil, why shouldn't Versus?
The reason most Final Fantasy spinoffs don't sell that much is because they are, simply put, not quality games, Kingdom Hearts was a quality game with a lot of publication, and Versus won't be different.
Now the main difference between KH and V13 is that KH got some hype due to the combination of Disney and FF characters, I assume it will be equal to the hype Versus will get due to it being a part of the Nova Crystallis blah blah.
In the end, yes, I can see Versus selling KH numbers ~5.
Bet with Dr.A.Peter.Nintendo that Super Mario Galaxy 2 won't sell 15 million copies up to six months after it's release, the winner will get Avatar control for a week and signature control for a month.
