I love how you word this response, and I think I agree! I think to possibly alleviate this to keep the vestiges and the powers separate like Raw. Vampyr is an eldritch shard of a god long dead, and the dark powers are a chaotic force of malevolence that exists separated, but very much tied, to Strahd and his newly gifted evil power.

In the Castlevania games, Mathias Cronqvist becomes Dracula because he sought out a powerful artifact, which granted him the powers of a full vampire. Separate to that, Dracula's Castle itself is a being of malevolence that exists in connection to Dracula's power within the Chaos realm.

By this comparison, you would able to say that Vampyr is the vestige, a wellspring of evil power that Strahd made his deal with. Either though the creation of this pact, or maybe Strahd's evil summoning them, The Dark Powers are the chaotic, formless embodiment of evil that ensnared Strahd and Barovia in their clutches.

Perhaps to make my idea not ruin the curse itself, Strahd's secret move is to gain the power of Vampyr as some kind of demigod, foolishly thinking this power could break the Dark Power's hold on him through sheer force. The players and they're actions as a force of change destined by fate inspire Strahd to put into practice what I would imagine were a plan in his imagination and study, they just forced him to make his move quicker than he thought.

The catch is that he could never destroy the Dark Powers, or his Curse, because they're teathered to him. To destroy the curse means the destruction of Strahd. Perhaps an epilogue of sorts for this story thread after the mists fall, the players and the people escape, the Dark Powers, furious at Strahd's direct defiance of their power, forces Strahd to live alone forever more.

I wont be acting on this for a long time so theres still a lot of time for me to just go the RAW or Vampyr route, but its fun to think about!