The answer, I believe, is when the committee chairman hurriedly adjourns the meeting. Tom turns to Vincenzo and says in Italian, "The honor of the family is secure." This tells me that Michael and Tom convinced Vincenzo to come to America because they told him that Frankie was going to dishonor the family by breaking omerta and testifying for the Law. But, by showing up, Vincenzo would shame Frankie into shutting up. It's also possible that Vincenzo's presence might threaten Frankie by implying that if he dishonored the family, Vincenzo would be obligated to take vengeance against Frankie's family.

Tom told the committee earlier that Vincenzo came to America "at his own expense to help his brother in his time of need." By preventing Frankie from going through with breaking omerta, Vincenzo helped his brother.