I don't have the book in front of me, but I remember that in the passage that Zaluchi is speaking, Puzo writes that not only was Zaluchi a Corleone ally, he was also very old school and against the drug trade. Having Zaluchi starting off on accepting a compromise was taking the laying the groundwork for compromising on drug trade, yet at the same time Puzo writes that his speech was the rambling of an old man saying the obvious, because everyone present knew that the drug trade would have to be approved by the commission and regulated (no matter how meaningless that "regulation" meant, it was a sop to the old school Dons)


I don't think that Vito realized he'd have to give on drugs at the meeting. The minute he called the meeting everyone would have known he'd have to compromise on the drugs. Though the Tatt's were seriously weakened and at the end of their rope, Vito was still negotiating from a point of weakness. Luca and Sonny, the muscle of the family, were dead and he was trying to save Micheal's life. Vito had to give on something to avoid fatally weakening his family through continuing the war and everyone at the meeting knew it.

That's why Vito realized it was "Barzini all along" at the meeting, Barzini took charge as the "impartial" negotiator and settled terms for Tattaglia, something that Tattaglia should have been doing if he was at all competent. Showing Tattaglia conferring with his adviser helped illustrate that, his presence at the conference was a formality since he wasn't the one "pulling the strings".