Vito never officially designated an underboss in the movie, though I think it's safe to say that Sonny was the de facto underboss. Early in the novel, Vito was worried about who would succeed him: Sonny was too impulsive, Fredo was weak, and Michael wasn't in the Family business. In a flashback scene in the novel (the one with the crooked boiler "repairmen"), Vito delegates Sonny to handle them because he was "thinking of making Sonny his underboss."
...and, of course, Sonny handled it in his usual heavy handed way that was discouraging to Vito.
This highlights several inconsistencies in the novel and films: no apparent underboss; when was Vito (or for that matter, any of his boys) made? Why would Vito tell his capos that they could someday form their own families?; Why only two capos? I'm sure that ya'll can think of others.
I felt the same way about them getting made. I think it is implied that they were but we just didn’t get to see the ceremony. So I don’t take much exception with that. This was in the 40s also so I think that the mafia had a real sense of being family driven and tight knit at the time as well which is why Vito only trusted Clemenza and Tessio to be his capos. Vito was not a greedy man like Michael ended up being. So I do not think he would have minded it if his capos started their own family eventually because he trusted them.
