Sonny doesn't have the brain -- uh -- for big deals except street things You're out! hothead Sonny

Vito's slipup
As regards that "Solozzo business" - It seems, Vito had kept both Sonny and Tom 'pretty much in the dark'

• Vito knew both Sonny and Tom were keen still doesn't excuse Sonny's out of turn blunder though
• not discussing his No decision with them prior to the meeting
• both were “totally in the dark” had no idea Vito had sent Luca to Tattaglias thus to Luca's death