Vader makes sense to me in the context of having the Empire’s fingers move into all areas of the galaxy and grasping for control. The Crimson Dawn would be the Emperor’s pawn in controlling the underworld through a puppet while he controls the more civilized world openly.
When Solo came out I had the same thought of it being Vader, having him control the underworld for the Emperor, maybe even doing it behind his back. But that way the post Order-66 era wouldn’t have two different opposing evil forces. You have the Galactic Empire who controls not only the government and the military, but has its own secret Jedi hunting force in the inquisitors under Vader, and then if Vader was in charge of Crimson Dawn they would secretly rule the underworld as well.