Sonny was not just a stupid hothead. In the book his character is more developed, and as Puso describes his actions after Vito is shot, he acts smartly and with very good knowlege of the matter.
He made mistakes, to be sure, but it doesn't mean that he's not at all clever.
BTW another mistake of Vito's was to send Luca to Solozzo. His devotion to Don was too well known, and Solozzo would be a fool if he didn't understand that he was sent on purpose, and didn't take advantage of the situation.