In the novel Sonny gets a call from a bookmaker stating that Vito is dead. In the movie the neighborhood people who gathered around probably thought he was dead or about to become so. Consequently, it was probably just people working from the knowledge they thin they had and not puposeful by the Tattaglias.
That seems most logical. Clemenza asked around, or was called, by the people in the neighborhood, and they all thought Vito was dead. In the novel, the detective who calls Sonny says "there's a lot of blood around but it's not as bad as it looks."
