Vito was a heavyweight no question. But he had to wait for Willie and Albert to be out of the way before he made his move on Frank. We all know Frank wasn't a killer and didn't like guns, so Vito knew he could push him over real easy. Joe had the political influence like Frank Costello. But unlike Frank he used to be an enforcer for Frankie Yale, his name has been mentioned with several killings, and he knew how to make money legally and illegally. So if Joe had become boss, knocking him off wouldn't have been easy.