Vito's tome sounds alot like predestination which I guess it is. But it's actually non-sensical because who knows what the destination is meant to be if meant at all.

Yes, if Fredo had been more competent, he could have upset Sollozzo's and Barzini's applecart. Vito wopuld have been alerted and Sollozzo would have been out of th epicture since Barzini would have considered him a liabilityat that point since the Corleones were still at their peak. More importantly, Vito would have remained in control and Sonny not. Fredo would have indeed earned respect for his defense of his father and playeda larger role i the family. Thus, Puzo would have had to rewrite his plot and FFC would not have produced a sequel.