Mike makes it clear to him that he is an important person (albeit with a price on his head) and probably wealthy. Once he demonstrated that he showed the proper respect and that he wanted to court his daughter in the Sicilian tradition, Vitelli realized his daughter had a chance to marry into money. I think the novel goes into greater detail about Vitelli checking out who Michael was.
"Io sono stanco, sono imbigliato, and I wan't everyone here to know, there ain't gonna be no trouble from me..Don Corleone..Cicc' a port!"
"I stood in the courtroom like a fool."
"I am Constanza: Lord of the idiots."
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