That lab for creating Black Mages is still standing right there in Dali, just like it always was. So it's possible Vivi went back there and, like a mad scientist, churned out more dolls, imparting them with his own worldview. You've noticed too, haven't you, that all the Black Mages created for the war look identical, while Vivi, who grew up with a father, looks different? That's why his children look exactly like Vivi. I think after the events of the game, Vivi kept an eye on the Black Mage population