Was it stupid?! though Michael undoubtedly goaded Greene still...

“Mike! You don't come to Las Vegas and talk to a man like Moe Greene like that!”

Greene: Yeah Let's talk business, Mike
1. First of all, you're all done
2. The Corleone Family don't even have that kind of muscle anymore
3. The Godfather's sick, right?
4. You're getting chased out of New York by Barzini and the other Families

What do you think is going on here? You think you can come to my hotel and take over? I talked to Barzini I can make a deal with him and still keep my hotel

Vito [Corleones] had no idea of Fredo's lifestyle! in Vegas
The Corleones knew that Greene had slapped Fredo around in public yet didn't bother to find out why Greene straightened Fredo a Corleone out?

I thought Vito kept tabs on everyone and everything [uh-oh before slipping]
Michael was blindsided that business was suffering! because of Fredo's philandering not because of Greene's skimming off the top