Vito did NOT "break his promise and code of honor", because the peace was not broken while he was alive, neither by him nor any of the other Families.

However he had to know that once he was gone, then others would most certainly move in to break the peace and whoever that was, their first logical move would be to assassinate Vito's successor...Michael. Therefore, he had to have advised Michael of what was to come, and plan accordingly. But make no mistake, every single murder committed on that baptism/massacre day was on Michael's order, not Vito's ... regardless of who 'conceptualized' the move.

Michael was indeed the Don that gave the order for these hits- No two ways about it. But that’s also because that’s the way that Vito set things up. Though Vito was aging and may have genuinely enjoyed planting in his garden, his retirement and turning things completely over was a convenient way of implementing his plan and removing himself at the same time.

I don’t think that Vito not being alive when the hits took place completely gives him a free pass. Can a code of honor be broken after one dies? I guess it would depend on the particular code. We’ll never know for certain exactly what the Don’s was- but we know that it was something very important to him. Whether or not you think he broke his promise, you must admit that he came awfufully close. Mary’s death may be a completely unrelated occurrence. Or, it may be an answer to the question.