Regardless of the fact that Vito may not have performed any specific act that broke the peace and thus his word, he fully intended to do so. The foundation and planning of the Potemkin Village which was inhabited by Mike's orchestrated acts of murder was constructed by Vito. It was no different than the many times that Vito instructed Tessio or Clemenza to persuade someone or make someone disappear. Vito's finderprints were all over the massacre.