"Is vengeance gonna bring your son back to you? Or my boy to
me? I forgo the vengeance of my son..."

I FORGO the vengence of my son. That line alone translates that in Vito's mind the killing of Bruno Tattaglia did NOT cancel out the attempt on Vito's own life, nor did it cancel out the taking of Sonny's life. Because if he felt that it did, then he would have never said "I FORGO" the vengence. Which makes me believe even more so that Tattaglia would never have been the one to agree that the attempt on Vito's life cancelled out the killing of his son. Tattaglia lost a son, Vito lost a son, but yet Tattaglia is even more worried that as time goes by and Vito's position becomes stronger, that he may attempt any individual vendetta for the killing of Santino. So if Tattaglia was really the one who originally agreed that the killing of his son cancelled out the attempt on Vito, than he surely wouldn't have been worried about Vito seeking revenge for the death of Sonny, because in truth, their both losing sons in the war cancelled out each other.

So it had to have been Barzini all along.