Intriguing question, DC!
As dt said, is it nature vs. nurture? I think it's "nurture." Circumstance, not country of origin of parents, put Tom where he was and made him what he was. Vito raised Tom not to be his successor, but to be a potential consigliere, or at least a bridge to the legitimate world--a guy whose smooth manner, eduction and conciliatory manner made him a good "public front" for the politicians, police officials (and movie moguls) that Vito needed to deal with. Vito sent him to law school, where he learned the art of conciliation. Vito taught him never to make a threat, etc. I believe that if Tom had been an orphan of Sicilian parents, and Sonny brought him home, and Vito raised him, he'd have turned out the same way because that's the way Vito wanted it.