It's important to keep in mind how the novel describes Luca's implementation of Vito's instructions. The novel makes it clear that such implementation took place over many months. How would Sonny have implemented those instructions? From what we know about Sonny, he would have been too impetuous. Otherwise, Q makes good points. In other threads we've discussed Luca's legendary loyalty to Vito and how Tattaglia and Barzini would not have been fooled. As Q points out, sending Sonny on such a mission wold have screamed deception.