I think that it would have gone this way:

Scenario A: Vito agrees to Sollozzo's idea

First, there's no hit because Vito agreed, presumably with regulations like they stated later at the Don's meeting (i.e. not by schools, kept to the black neighborhoods, etc). Barzini consolidates his power, because Barzini also has major connections just not as many lawyers/judges/politicians like Vito does. I think the end result here is that there is peace for longer (potentially much longer) but eventually Barzini can't control his ambition and something happens down the road. Now both the Barzini family and the Corleone family have gotten fat from the drug trade, so a war would likely be more brutal and more open. However, the Corleone Family at this point would have gambling, the unions AND the drugs. So I think they end up ahead as they were already the strongest family, and the money from narcotics helps them grow stronger. Michael almost certainly does not end up as the Don but probably moves away and has his whitebread family with Kay as a mathematics professor.

Scenario B: Vito refuses Sollozzo, Sonny doesn't open his mouth

This becomes interesting, because we don't know for sure what Sollozzo would have done in this case. He finds no ch*nk in the Don's armor, no hot-headed eldest son who would inherit the family if the old man dies. We know for a fact Sollozzo cannot take no for an answer, because he is a man of respect and Don Corleone's "no" means Sollozzo cannot operate his business (technically he could, but it would be at an extremely great risk without the lawyers/judges/political connections of Don Corleone), and he is not the kind of man who will let another man dictate his life. In this scenario I think Sollozzo goes to Barzini (I am not sure if at this point in time Sollozzo was talking to Barzini, or just Tattaglia; we know it was "Barzini all along" who set the seeds for the hit, but we don't know his exact involvement), who also presumably has some politicial/legal connections, and does business in a lesser scale than he originally planned, but there is no hit as it's too risky.

I think the end result here is exactly what Tom says when he suggests Vito agree: Well, I say yes. There is more money potential in narcotics than anything else we're looking at now. If we don't get into it, somebody else will, maybe one of the Five Families, maybe all of them. And with the money they earn they'll be able to buy more police and political power. Then they come after us. Right now we have the unions and we have the gambling and those are the best things to have. But narcotics is a thing of the future. If we don't get a piece of that action we risk everything we have. Not now, but ten years from now.

Barzini and maybe Tattaglia, even Cuneo and Stracci get into the trade, and "ten years from now" I think Barzini makes his power play. All hypothetical at this point, but I think in this scenario the Corleones are on the losing end. Things happen (maybe Sonny still gets whacked), the Corleones sue for peace, and Barzini becomes the most powerful family. Michael probably does not end up as the Don, but could depending on circumstances. I think in this case though he does not, and the Corleones become a minor family.