Vivenna Saves Vasher—Kind Of
Vivenna has a few things going for her here. First off, Denth has gotten rid of his Breath. He doesn't want to have it as he tortures Vasher. It made him too aware, too pained. Being a drab as he does it is easier for him. With Tonks dozing, that means that nobody in the room has enough life sense to notice Vivenna hanging outside.
Secondly, Denth doesn't really like what he's doing. He feels he took Vasher too easily, and the torture isn't satisfying. He'd much rather kill Vasher in a fight, as he later realizes. So there's some hesitance to him in this scene, as you might notice. He doesn't just stab Vasher or Vivenna. He goes to free Tonk Fah, then hesitates before turning back and challenging Vasher. Denth was actually hoping that something like this would happen. (Plus, he does care for his friend Tonk Fah. Again, Denth is far from purely evil, no matter what he would like people to assume.)
Denth is the better duelist. Even if Vasher hadn't been beaten and tortured, Denth would have won. Except for the trick Vasher was planning, which Vivenna interfered with. But we don't know about that yet . . .
I don't know if you remember that Vivenna put a whole bunch of Breath into Tonk Fah's cloak accidentally, but it happened during the time when she found Parlin. It might be just a little bit of a stretch here, as I don't know that people will remember it. As I consider it, I should have mentioned what she'd done one more time.
Also, I hope that you don't mind the line that goes something like "Vasher is plunging to his doom from a three story window—of course he'll live!" It's a little bit self-aware, and I'm not trying to break the fourth wall. Denth has simply known Vasher for a very, very long time, and knows that something so simple isn't likely to kill his old friend. That, mixed with Denth's penchant for sarcasm, produced this line.