Vader wins if this is canon. He's an elite duelist and is probably the 3rd most powerful force user after Sidious and Yoda.
Legends is a different story. Like I said earlier, it depends how the fight goes. If Vader uses his superior force powers to his advantage, he could win. But if it's more of a lightsaber duel, Mace probably wins.
Yea, Canon Vader has a stronger argument than Legends Vader. In legends, Vapaad basically gives Mace the win as it negates Vader's advantage here, power.
And yes, Mace would retain the inner darkness he recieved before his bout with Palpatine. As we've seen, the inner darkness Mace recieves from what he confronts is not something that dissapates after said confrontations:
Obi-Wan's heart clenched. This was not the Mace Windu he knew and admired; it was as though the darkness in the Force was so much thicker here on Coruscant that it had breathed poison into Mace's spirit, and perhaps was even breeding suspicion and dissension among the members of the Jedi Council.
The greatest danger from the darkness outside came when Jedi fed it with the darkness within.
He had feared he might find matters had deteriorated when he returned to Coruscant and the Temple; not even in his darkest dreams had he thought it would get this bad.ROTS(@lordofthelight)