I think sometimes Vito is looking ahead. For instance, when Tom says getting into drugs is important because they'll be the big thing in ten years, Vito may be thinking that he wants to be free of a lot of what they do in ten years. So Tom the lawyer might be the guy who Vito thinks might be best suited to transition the family to the next generation.

Also, Tom is smart, and therefore is a Michael substitute. As TB points out, the family has a crisis of leadership and succession. At least as the book/movie opens, Vito's seeing if he can yoke Sonny and Tom together to run the family in the future, each contributing something the other doesn't have.