I don't disagree that Fredo dropped the ball in las Vegas. He proved that he was not capable of doing what he was supposed to in the Casino business.

But the Corleones did NOT really send Fredo out there to learn the Casino business. They sent him out there to recover from what he witnessed when his father was almost killed. To protect him because a war was about to break out. It was a business arrangement in the sense that Corleones agreed to finance Moe's Vegas hotel and they arranged to have the Molinari family provide Moe with what he needed for his Hotel. Their price : guarantee Fredo's safety.

The Corleones never intended for Fredo to actually learn the casino business. It was purely a business arrangement made so that they would not have to worry about Fredo's safety during a war.

Had Sonny lived and went out to Vegas and was given a reception like that by Fredo, you can rest assured that Sonny would have kissed Fredo on the head and then would have started banging those cocktail waitresses two at a time himself before meeting with Moe.