I have found the tweet posted by Solange Keoleyan, a writer for Violetta, stating Vargas is Leon's last name:
So can we finally say Vargas is legit his last name?
Second, isn't Francesca's last name spelled Cauviglia, as stated by Marco?
I know most of the disagreement comes from the Spanish version when he uses the third pronunciation of u to spell it, making it sound like it is a U V, when it is just a U. But since everything in this wiki is based solely on the English version, it is clearly stated to be a U and a V, thus spelling Cauviglia.