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Obviously, by the time Vito tells Tom to keep an eye on Rocco, everything has been turned upside down and Michael must become the new Don. It's possible, though, that this curious remark from Vito arises from conversations with his personnel manager Clemenza (who valued Rocco's "good judgment") about young men outside of the inner circle who could be groomed for leadership in the next generation. Clemenza himself need not have been told that head of the Family was among the positions that Vito was trying to fill.
The novel also has Tom expressing surprise at Vito's appraisal. He states that Rocco had not manifested any exceptional behavior; he had acted as any chauffeur should. Thus, Tom attributes Vito's appraisal to the Don's keen eye.
