Anders, no contest. He has a fervor to his fighting that Vivienne lacks, plus I can't imagine Vivienne has been in nearly as many tough, desperate combat situations as he has. His experience would let him dominate the fight, Knight-Enchanter or no. And to add to that, his bond with Justice has proven to let him endure wounds that would cripple anyone else, at least for a time.
Anders and Justice/Vengeance V Vivienne? Anders, no contest... Partly because Vivienne would absolutely just dismiss him as shabby, and not very powerful, because of his looks (she does that). But mostly, because Anders is powerful himself, and add to that Justice's power, Viv would be obliterated in a blink, knight enchanter or no (she has no mana stopping/damping powers, after all).