Good question.

Its believable that the Corleones wouldn't have had to resort to making the actual threat.

They may have worded it so, on the surface, it appears that they required Vicenzo to come to the hearing hoping that his presence will lead to Frank feeling shamed and therfore cease in his treachery towards Micheal.

However, I think Vicenzo would have known, as did Frank, what the repercussions would have been if Frank didn't suffer this bout of 'shame'. More of an unspoken threat as I'v mentioned before.

One thing this thread had led to me question though is this: why didn't Frank anticipate that Micheal would target his family? Sure theres not a lot he could have done about it but he probably wouldn't have even got as far as the hearing and thus wouldn't have been forced to commit suicide.