Vito was unlucky to have NYC mobsters rat on him for an old murder.
Maybe he shouldn't have commited that murder in the first place, but that's an entirely different discussion. Anyway, he rightfully paid for his crimes.
Blood and murder is bad for business in general and that's why the Violis and Hamilton are behind this. Because vendetta has no price.
I think it would have happened nonetheless. The Rizzutos were severely weakened by operation Colisee. And because like you said, it was a vendetta.
