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 man in my position cannot afford to be made to look ridiculous!"