Consider that Vito “gave his honor that he will never seek vengeance and that he will leave with a pure heart” – presumably to mean the promise was as pure as his heart.

Arguably, Vito’s pledge to not seek vengeance may have been broken the moment it was said as the vengeance seeking plan laid out for Michael (quite possibly created before or even at the meeting) was riddled with vengeance conceived of by Vito. If we generally agree that Vito was complicit, we can conclude that he sought vengeance. I think that this would qualify as a pledge breaking.