This brings up a question I've thought about.

What would have been Vito's reaction to the killing of Fredo? Michael would have waited until Vito died before killing Fredo in all likelihood.

But if Vito were living and aware of the Tahoe attack and Fredo's involvement with Johnny Ola, could he have approved of Michael's actions?

I think Michael's decision to kill Fredo would have horrified Vito despite Fredo's betrayal. Sonny, I think, would have pulled the trigger himself.