The only to master Vaapad and not fall to the Dark-Side.
Which in my opinion, says something about his will against the Dark-Side.
He didn't turn when fighting Palpatine even though, for sure he had to been overwhelmed by the power of the Dark. And thats even controversial, some think he did turn hence his wanting to kill Palps.
I think if he did kill Palpatine, he would have fallen or come close to it. So Anakin may have saved from being an agent of the Dark Side. But I don't think Mace would have fallen. His will is too strong. I believe his use of Vaapad gave him a resistance to the influence of the Dark Side. But not many Jedi could engage Palpatine in a duel without succumbing to the darkness. So that is a testament to the will that Mace Windu had.