In the meeting with Sollozzo, Vito says that he knows Sollozzo is a "serious" man. I think that's a nice way of saying 'I know you're a bully and you aren't going to stop until you get what you want'. Vito may be more cautious for good reason. He can't be sure of how far Sol is willing to go. Sonny is short-sighted and only thinking of the money. It's an interesting look into the chess game of gangsters, who in reality are clever, ruthless business men.