The novel said, "Although they [Clem and Tess] never spoke to anyone [about Vito killing Fanucci], the whole neighborhood, within a few weeks, also knew. Vito Corleone was treated as a "man of respect" by everone."
True TB. It also states that Tessio suggested to Vito and Clemenza that they could take over Fanucci's customers. In addition, it states that part of Vito's murder strategy was to have witnesses "to show that he [Fanucci] had left the Corleone home safely". That Fanucci ended up dead soon afetr and that Vito murdered him didn't require much math from the neighborhood's inhabitants.
