Não é declarado explicitamente no filme e o romance se refere apenas ao fato de Vito ter prestado um ótimo serviço para Don Tommasino
Michael had been transported from the ship docked at Palermo to the interior of the Sicilian island, to the very heart of a province controlled by the Mafia, where the local capo-mafioso was greatly indebted to his father for some past service.
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The capo-mafioso was a man in his late fifties named Don Tommasino
The Godfather - Mario Puzo