Vito didn't have an underboss per se. I think Sonny was an understudy, not really an underboss. In fact, Sonny's actual position and duties in the family were never made clear (although, apparently, he did have a regime). Following Vito's death and the Great Massacre of 1955, Michael didn't really need an underboss--or a consigliere, for that matter. Michael, the hypercontroller, trusted in no counsel but his own.