At Vito's funeral, Michael and Tom learn that Tessio is the traitor, that he is setting Michael up to be killed at the meeting with Barzini and the other dons. So why does Michael tell Tom he's going to meet with them after the baptism when he doesn't plan on doing anything of the sort? Especially given the fact that Michael would have to be a fool to agree to any kind of meeting after learning about their plan to kill him at the one that was being arranged.