It wasn't Michael's decision to send Fredo to Vegas--the decision was made by Sonny, as acting head of the family. Michael had suggested, after Tom was released by Sollozzo, that Fredo be included in the decision making. Sonny said no, and he was seconded by Tom (who, in the novel, said, ..."leave him out of everything..."). What's more, Fredo's palship with Moe Green started while Michael was in Sicily. Moe may have given him ideas, but I agree that Since Sonny was Fredo's older brother, Fredo probably wouldn't have plotted against Sonny.

But when did he start plotting against Michael? At the time Michael visited him and Moe in Vegas, Fredo looked happy as a clam, carefree (and stupid) as ever. Yes, he took sides against Michael, but I don't think it was part of a plot--yet. But by the time of Anthony's party (some years later) Fredo was a lot less carefree: henpecked and humiliated by his wife, not included in the family business, running a whorehouse (and some Mickey Mouse nightclub). The seeds of resentment were there--big time. I have no reason to doubt that, whether or not Moe planted the idea of betraying Michael before Moe was killed, Fredo was good and ready for that overture from Johnny Ola in Beverly Hills.