SPOILER:
In the novel, in the scene where the "boiler inspectors" try to shake down Vito, he tells Sonny to handle it. It was a kind of test for Sonny because Vito was "considering making Sonny his underboss."
True enough TB. But "as" is quite different from "considering", right?
Also, it supports the argument that Vito had no underboss, since Vito's consideration of Sonny for that position is not referred to as a replacement for the existing underboss not does it reference any other candidates for that position.
