As we do regarding so much of the Board's content, we speculate about who made the Vegas decision. I imagine that the Don had Vegas on his radar screen for years and would have proceeded in that direction had events not overtaken him. Those events placed certain decision making in Sonny's hands for a time. It was probably Sonny who decided to move Fredo out to Vegas as a way to introduce a Corleone presence in Vegas and to utilize Fredo in a fairly innocuous way.

As for Sonny having to die, I don't get it.