I agree with everything Turi said. Plus, Hagen was trying to keep Vincenzo from being forced to testify. Questad says, "Will he come forward and be sworn?" He's asking, because Vincenzo wasn't on the agenda, and wasn't subpoenaed. By replying, "He is not under subpoena, and his reputation in his own country is impeccable," Hagen was saying to the Committee: "You have no legal right to put this guy on the stand. And, if you decide to issue a subpoena, I want you to know that the guy is a citizen of another country, where he's respected, and he's not a gangster or part of this inquiry."
Some people here believe that Michael threatened Vincenzo to make him come to the US, perhaps by menacing his family in Sicily. I don't. I think Michael convinced him that his brother was about to bring disgrace upon his family--but if he showed up at the hearing, his appearance would shame Frankie into doing the right thing.