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.

I still think if Fredo thought he was an imoprtant member of the family he would have not betrayed Michael. Deep down Fredo knew he would not hae made a good Don, znd he just wanted some "respect."

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

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.