Tom refers to Vito as "your father" both when talking to Mike about his future,as well as when he tells Sonny "If your father dies,you make the deal". Also when he goes to tell Vito about Sonny's death,Vito says that Tom should "tell your Don" (not your father)what everyone already seems to know.It was understood that Tom was as close to the Family as any non-blood relation could be,but he was never given official (i.e. adoptive) status. The novel goes into this a little more.