This raises another question: When did Vito realize that his true adversary was Barzini, not Tattaglia?
It seems, from the "could never have outfought Santino" comment, that it was only later on that Vito began to look beyond Tattaglia.
So, if Vito had said yes, and no assassination attempts were made, would he have remained oblivious of Barzini's machinations against him? Would that have given Barzini an advantage and enhanced his chances of deposing Vito?
A good question, partly because we don't know how "looped in" Vito was while he was recovering.
One thing I think is that we sometimes misread the line, "It was Barzini all along." We focus on "It was Barzini...," but I think the key is "...all along," so that Vito is telling Tom that Barzini is not just the main adversary, but the one who set the whole thing in motion. (Edit) I guess that would imply that Vito wasn't particularly suspicious of Barzini at the time Sollozzo entered the picture.
