We know from the real time history of the mafia that warfare among the families and the attempts of one Don or the other to be pre-eminent is usual and customary. So, I don't think it was any different in the GF. Barzini wanted that pre-eminence regardless of what part the drug business played in it or didn't play in it. To answer Woltz, Vito was probably not oblivious to any of the Dons coveting what the Corleones had, but what could he do about it? As far as we know, Vito was doing what any Don would do to preserve what he had. He could have entered the drug business well before Sollozzo made his proposal, but he decided not to. Were there risks to that? Probably. What Vito did not calculate was Barzini's efforts to sow animus among the Dons toward Vito for Vito's refusal to participate in the drug business.

It reminds me of Vito Genovese's efforts to discredit Frank Costello in the Mafia community and thus undermine his leadership.