A standard one is destroying the parent vampire (possibly within a short time window), but I also might want to keep Dracula around for later.
Yeah, but Dracula always comes back. "Killing" him releases his immediate thralls and disintegrates him into vapor which dissipates, only to reform sometime later as part of his contingency plans.
Or you could do total blood transfusion: all the bad, vampiry blood out; all new, good human blood in. That's how they did it in the movie Near Dark.
Option 3: trial by fire (not literally... maybe). Have the vampire servant trapped in close proximity to something that normally repels vampires: locked in a closet full of crucifixes, submerged in holy water, or even tied down on a roof as the sun comes up. And, yes, even surround them with an encroaching fire. The sunlight one might kill regular vampires but we can assume servants are not full vampires and, like these other methods, it may simply hurt them (badly) and eventually force the vampiry part of them to "leave."
Also, if you have magical PCs/NPCs, you could do it as a spell (created or found) that literally separates the vampire from the human. You get your human back and an insatiably bloodthirsty spirit of evil for your PCs to fight. Win/win.