Vito was slipping If anybody is to be blamed, first and foremost, it is Vito for appointing Tom as consiglieri
Michael's concern was justified and in times, detrimental too

Tom had strange wealth of information, Fredo was talking to him yet another failure of his instead of finding out what he's got under his fingernails, I reckon Tom says gleefully! payback time!!
“No way we can get to him. If we don't it's up to five counts of perjury”
"Fredo says he doesn't know anything I believe him"
“Roth played this one beautifully” ”

Now I believe it was Tom who opened the drapes in Michael and Kay's bedroom!
Fredo is a pimp He never could have outfought Michael but I didn't know until this day that it was Tom all along!