This is dependent on how much some accepts interplay between the book and the movie: I don't think Tom would be blamed for not restraining Sonny. During the wedding, Vito sees Sonny chasing skirt instead of attending to business, and Vito thinks:
It was hopeless.... If he refused to be instructed, Santino could never run the family business, could never become a Don. He would have to find somebody else. And soon.
Within that context, it seems clear that Vito wasn't just sparing Tom's feelings when he told Tom that Sonny had been a bad don. It also seems likely that at some point in Michael's years of training Vito told him that no one could talk to Sonny, and that his death was on him.
