The Emperor (2-147L) and Vivi (3-149S)

I have a question regarding summoning the Emperor(2-147L) and Vivi(3-149S).

Can my opponent use Vivi's ability as the Emperor is coming on to the field?

IE. I pay 4 crystals, declare I'm summoning the Emperor(2-147L), my opponent, as a response, chooses to activate Vivi's ability and break a forward currently on the field due to his damage.