Certainly not Fredo.

As we learn in the "fishtank" scene, Clemenza says Vito promised Tessio and him that they could "form our own family" after he was gone. I emphasized "our" because it imples a dual Donship. And that would have been a fatal dilemma for Vito: there's never been a dual Donship. Plus, I doubt that either Tess or Clem would have accepted being an underling to the other. They'd either have fought to the death for supremacy, or divided up Corleone territories among them. Either scenario would have been fatal to the Corleone family.

As for Vito's threat ("I'm going to blame some of the people here"): I think it was a threat of muscle but more of withholding police/political support of the drugs business. Another war, coupled with withdrawal of support for drugs, would have decimated the families even if Vito lost.