Even Moe Green thought the Corleones were being run out of New York by the other families, and that Barzini was now the Don with whom he needed to deal. Vito knew that the Corleones were (wrongly) perceived as weak, and he also knew that someone within the family would betray Michael to Barzini. It was not a stretch for Vito to understand that once he died and Michael was officially head of the family, Barzini would rub him out and take over. Given that there would be a traitor, it makes sense that the traitor would not let on to what he was doing until Michael was out of the way. It came as no surprise to Michael when Tessio approached him about how the meeting could be held on his territory where Michael would be "safe." Vito, with his Sicilian cunning could have easily figured this out. If there is directorial license in this I think it is to tell the viewing audience of what was coming. My guess is Michael had already figured all this out on his own, and was humoring Vito by allowing him to give him this warning.