I've always believed that Sol approached Barzini first. He knew Barz was anxious to move past Vito as Numero Uno, and would have the most to gain from the riches he'd make with Sol through drugs. But Barz told him, "If Corleone knows I'm your partner, he'll say no because he'll know that you'll strengthen me. Go to Tattaglia. He's a pimp, and Corleone doesn't fear him. Promise Tatt and Corleone whatever you want, but I'll be your real but silent partner." Barz couldn't lose: If Vito said yes, Barz would get richer and stronger secretly. If Vito said no, Barz knew Sol would have to kill him, which would make Barz Numero Uno even without the drugs profits.

So, what if Vito'd said yes? The novel says Sonny was itching to have a big operation of his own, and to get out from Vito's thumb. But Vito wouldn't have trusted the hothead to run a dangerous business. And in any event, Vito needed to keep arms-length from actual operations, and having Sonny run his end wouldn't exactly be arms-length. I'm guessing he would have delegated it to Tessio--the novel says he kept Tess on a long leash partly so that his foes wouldn't readily see that Tess was one of his capos. But such an arrangement would have put Tess in the path of temptation--always a bad thing for any Mafioso, no matter who he was nominally loyal to. So, I'm guessing, Barz eventually would have shown his hand to Tessio and conspired with him in the same way he did to get rid of Michael.