I would not blame Vito for this because 1) He thought Santino would be Don and once he died Vito, who was old and sick had to groom Michael, and simply didn't have the time to deal with Fredo.

That Michael and Tom never saw Fredo as a potential threat is their fault.

Good points. There is no way the Don could have foreseen the chain of events that led to Fredo becoming a turncoat. However, as a Father who deeply loved his family, and took great pains to assure their futures were to be secured long after he was gone, he failed Fredo. As you point out, likely a consequence of having to bring Michael up to speed in such short order. Perhaps the lesson learned is that the Corleone family, during the period we are speaking of, was only as strong as the weakest brother.

Here's an example of what I am talking about. Whoever hatched the plan to set up Geary, it was done in a place Fredo ran.
He was clearly in on the plot, and there is no reason Michael and Tom could not have lavished him with praise, given him credit, and telling him he was the family's "secret weapon."
Yes, I understand this happened after the betrayal, but thing like that being done all along could have kept him in line. [/quote]

Great idea. Fredo was so starved for attention that he would have believed that he really was the secret weapon-- much the same way he was proud to be learning the casino business.