As for the purpose of Vincenzo's presence in the courtroom, I believed for many years that it was a threat to Frankie - we kill your brother if you testify. Michael's "It was between the brothers," was just one more lie he told to Kay.

However, after watching and listening more carefully, I no longer think so. I believe that he was there to remind Frankie that they took an oath, and that he would disgrace their family if he broke it. To me, that is supported by Tom Hagen, after Frankie changes his testimony, telling Vincenzo (in Sicilian) that the honor of the family was intact. This makes Michael's statement to Kay sincere after all.