Princess Rizelle's question immediately made Nicien recall that attack and Vincent. He thought about it but... he didn't think he could come clean about Vincent and that assassination attempt for now. Maybe because he believed that even if Rizelle could understand other things, it would be like impossible to expect her to understand that attack and the fact that Vincent was still alive.
"When Avilina came for me and told me the truth, I was only 10," he told the Princess. "When she fled twenty years ago, she spent several years seeking out and regrouping those military officers and soldiers from the Varian Army who are still loyal to our father. I heard that most of them flee from the capital under the order of our father and lie low until Avilina sought them out. Now, those people still exist under the name 'Varian Guild'."