Durkon turning his vampire spirit into a copy of himself is like the opposite of rules lawyering, this isn't something that's within the scope of the rules of base D&D, if it was happening in a real game it'd be totally within the realm of either the narrative of the story and thus not touched by rules at all, or completely homebrew. In OotS it's something that happened because the narrative of the story needed it, with the homebrew explanation of it being some sorta rules based thing only existing lightly because that's one of the themes of the story being told, that they exist in a very gameified world. But don't get it wrong, it totally happened because it needed to happen for the story, not because it was the native consequence of a series of rules the writer had for the world. That's essentially the opposite of rules lawyering.
As for deus ex machina, I think that's a bit more a personal sorta feeling thing. For me, personally? It seemed like this was fairly built up, ever since we learned that Durkon still exists inside his own mind there was the possibility that he somehow regain control or at least distract the vampire at a critical moment, and once Durkon made the deal for one last vision it was pretty clear that this was going to be his big moment. And turning the vampire good (or shocking it emotionally for a big distraction) seemed in the cards since page 1126, 4 pages ago. (Although obviously that build up is easier to see in retrospect) This didn't come out of nowhere. The highest willpower character in the group, potentially in the setting, managed to overwhelm an evil presence in his mind with his willpower during a conversation they've been building up for a very long time. For me, that fails to make me think it's a deus ex machina, but your mileage may vary. It's not like he sat on a throne of alignment changing or something though.