Lewis doesn’t take a clear side in the disagreement between Vinny and Danny (over whether most subprime mortgage bond traders are simply blinded by their own self-interest or actively trying to commit fraud) perhaps because it is something that’s very difficult to prove. Perhaps the point is that, from a distance, obliviousness and fraud look very similar, and the difference may not be meaningful—after all, the result (the market crash) was still the same.