If Michael had died along with Appolonia, there would never have been a meeting, so Vito would not have known that it was Barzini all along.
I think Vito would have known someone bigger than Tattaglia would have been behind Michael's assassination. Tattaglia, as we know, was considered to be nothing but a pimp by Vito. That's why he sent Luca on that suicide mission, to ferret out, or at least point to, the real power behind Taggalia. He know instinctively that Tattaglia was too small-time to simultaneously back Sollozo's big heroin racket
and try and assassinate him.
Personally I don't think Vito would have steeped away if Michael was killed, especially after loosing Sonny. I think though you're right in that Vito would have annihilated the Tattaglias but would have eventually been killed by Barzini, who would of had, by that time, more resources than the depleted Corelones. Or who knows, Vito could have gotten lucky himself and killed Barzini too.
