Very tough call, and the lack of information about Diamente and his fighting style makes this difficult. My impression is that Vergo would be in the best position to win. He's got incredible speed and a powerful offense, not to mention an incredibly solid body that took beatings successively from Sanji, Law and Smoker without breaking stride. It's possible that Diamente could manage, but it seems to me that he would have an uphill battle.

I think Diamante is stronger than Law, and Law beat Vergo, so..

Law beat Vergo while Diamante is a top executive.

Wasn't Vergo Doflamingo's right-hand man? He is above Diamante.
Law could defeat him only because he had that specific devil fruit (that could dismember him instead instead of bringing him down) and he had to team up with Smoker for a while.