DC, I humbly disagree. I think that turning point came for Vito a bit later. I think he was even willing to lose his job and not be turned. However, when Fredo is sick and he feels so helpless, and Clemenza offers him the hijacking job, and then Fanucci insists that they pay tribute, I think THAT is his turning point. Remember, in the book, he is shocked that Tessio and Clemenza are willing to turn over a share of the money so easily. He is the only one smart enough to realize that Fanucci is a blowhard with no real power or influence, and can be eliminated quite easily.