Sonny wasn't totally stupid. He made a very wise, strategic move in sending Fredo to Vegas after Vito was shot. Fredo suffered an apparent nervous breakdown. With the family at war, Fredo would have been, at best, a distraction to Sonny and Tom; at worst, a very vulnerable target for the Corleones' foes. By sending him to neutral Vegas, under Don Francesco's protection, Sonny removed a serious danger to Fredo and to the rest of the family. He also "planted a flag" in Vegas by putting Fredo in with Moe Green, whose casino the Corleones had bankrolled. It's one thing to finance a casino, another to have your brother inside.