And what was that completely unaided walk down the hall all about? Seeing as the Don has been quoted as blaming Barzini for everything, surely the guy would be surrounded by more than one guy and a driver?
It was timely and credible. Vito deliberately made himself appear weak at the Don's meeting, and Michael made himself appear weak by not fighting Barzini's incursions on his territories and by agreeing to a meeting. Both of them accomplished what you saw: they fooled Barzini into thinking that the Corleones were finished. He had nothing to fear from them, hence only one bodyguard.
