A little more context might be helpful to you:
Puzo drew on many real-life situations in the novel, particularly in sketching Vito's early Mafia years. Vito himself is a composite of real-life NYC Dons. Their identities have been discussed on this site many times. Try here (scroll to the top):
http://www.gangsterbb.net/threads/ubbthr...=true#Post36689The incident you referred to was based on the real-life NYC Mafia Castellemmarese War of 1930-31, between Joe (the Boss) Masseria and Salvatore Maranzano. It gets confusing in the novel because Puzo transposed some characters. In real life, Maranzano won by enticing some of Masseria's henchmen (especially the young Charlie Luciano) to betray him. In real life, Masseria was lured to an Italian restaurant, then riddled with bullets while eating. In the novel, Puzo somehow has Vito in the Maranzano role, and calls the Masseria character "Maranzano." So, to better understand this scene, think of Vito (in this case) as the real-life Maranzano, and the novel "Maranzano" as the real-life Masseria.
As Olivant said, in real life Masseria did ask for Capone's help during the Castellemmarese War. But it is not recorded that Capone sent gunmen to help him, nor that any Capone gunmen were chopped up. I think he just sent money.
The Irish gunmen Puzo refers to also represents a real-life situation:
Following Maranzano's victory in the Castellemmarese War, Maranzano rewarded Charlie Luciano by putting him in charge of Masseria's organization. But then Maranzano made a fatal mistake: he called a meeting of 600 NYC area Mafiosi, declared himself
capo di tutti capi, and demanded a monthly tribute from all of them. This didn't sit too well with the younger guys, so they asked for Luciano's help. Luciano asked Meyer Lansky to recruit a hit squad. The burst into Maranzano's office, identified themselves as "Treasury agents," and demanded to see his books. While he complied, they shot and stabbed him to death. While they were fleeing down the stairs, an Irish gunman--Vincent (Mad Dog) Coll--was waiting for an elevator to take him to Maranzano's office. He'd been hired by Maranzano to kill Luciano. That's how close Charlie came to being whacked. It's one of the reasons he was called "Lucky."
