I think the drugs business was a force greater than Vito, Tom and Genco (although, in a different thread, I argue that Vito could have delayed the drugs business over the short term by killing Sollozzo). Tom gave his Don logical advice about the money potential of the drugs business, and the negative implications for the Corleones if they didn't get into it. He was right. Vito saw the threat to the entire Mafia posed by the drugs business. He was right. But it was his decision, not Tom's; and it would have been his decision, not Genco's, if Genco had still been consigliere when Sollozzo came a' callin'.

As for terms of an agreement: The Corleones obviously would have been far better off if Vito had agreed to Sollozzo's terms because Vito ultimately agreed to share his police and political contacts anyway. If he'd said yes right away, he would have gotten a share of the drugs profits instead of some fee for renting the cops and pols. He would have been dealing from a position of strength, not weakness. And, most important, there'd have been no war, with all its disastrous results.

But it raises a much larger question: how does an ultra-powerful man like Vito reconcile two irreconcilable forces--the inevitability of drugs due to the money potential that's irresistable and has greater potential power than even he wields; vs. the inevitability that drugs will destroy his family and all the others in the Mafia? That's worthy of a Greek tragedy.