Pete's guess is a good one, My guess is that Vito never formally became a citizen because he didn't have to. Under immigrant regulations that were in effect until 1924: If you could get to America on your own, enter legally and be processed at Ellis Island or another official immigration center, and not pose a health or security risk, you were in. You could stay forever without becoming a citizen. Sure, Vito could have leaned on one of his politicos to arrange citizenship, but it would have raised his profile and made him vulnerable to losing citizenship if he were convicted of a crime.