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.
I always assumed that Sonny was underboss because he stepped up to the plate once Vito had been shot. Unless, Vito had made it clear that he wanted Sonny as acting boss.
