I agree with all.

I think Michael dispatched Tom to Sicily to inform Vincenzo about his brother’s infamia, and to persuade Vincenzo to come to Washington to “aid his brother in his time of need.” I infer that Vincenzo was a gabellotto, from Frankie’s description to Tom: “He’s ten time tougher than me…he could have been big here, he could have had his own family.” So, Michael (through Tom) didn’t have to threaten Vincenzo—he would have seen immediately that when word of his brother’s betrayal of omerta and his own Don reached Sicily, Vincenzo’s hold over his “two meal town” would weaken, and his enemies would stop at nothing to get rid of him, even, as LucaB said, kidnapping or killing his grandchildren. I bet Tom didn’t have to work hard to convince Vincenzo to come to Washington.

As Pete astutely pointed out: Frankie was already wavering about his testimony before Vincenzo appeared: The first thing we see him saying to the FBI guys is, “Ten to one -- ten to one shot you said -- ten to one shot he would take the Fifth -- and I lose.” Sure—all the mobsters that the FBI guys hauled before Congressional hearings took the Fifth, so they told Frankie that if Michael did, he wouldn’t be needed as a witness, and his survival and betrayal of Michael would be kept secret, at least for a while. Credit Roth’s evil genius for knowing that Michael’s obsession with “legitimacy” would prevent him from ever responding to accusations of his criminal past with “I decline to answer that question because my answer might tend to incriminate me” [emphasis added]—especially before a televised hearing. Hence the brilliant perjury trap Roth set through Questadt and the committee chairman that Michael fell right into.

And as Lou said, it all came down to Sicily, honor and family. Once baleful glance from his brother and Frankie recovered the Tradition he grew up with and lived by. As Tom said to Vincenzo after the chairman adjourned the hearing: La honore di la famiglia sta posto—“The honor of the family is intact.”