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."