In the novel, Vito tell Michael that maybe he hasn't got the heart anymore to do the things that Michael is about to do. Mike tells Tom that Vito planned alot of it, that he didn't realize how intelligent he was. So, yes, Vito was involved, but, technically, he kept his word about not breaking the peace and that included Tom. Also, as Tessio makes clear, Barzini invaded his territory and has given Michael provocation. So, it is Barzini who has broken the peace.

"Generosity. That was my first mistake."
"Experience must be our only guide; reason may mislead us."
"Instagram is Twitter for people who can't read."