OP: So, will suddenly adding this species to the mix over twenty chapters in throw the reader off (in a bad way)?
It certainly can do that, but you might get away with it. One way is to spotlight the fact that you haven't previously mentioned them, if you can think of a reason nobody has mentioned them in the first twenty chapters. Perhaps some magical concealment, or vampires are just covert. Perhaps they have learned to conceal the fact that they are operating in an area, disguising their kills as something else: rape and murder or lethal mugging. No characteristic teeth marks on the victim's neck. Or perhaps they have acquired magical amulets that transform the punctures of their teeth marks into the appearance of run-of-the-mill strangulation, looking like a rape. Or after feeding, they can make the remains of their victims vanish.
You can even begin the chapter with the vampire telling somebody (say her date) all the reasons she never believed in vampires, all the ways vampires don't make sense.
Yet another solution: In the final scene of the previous chapter, the previous POV character by some magical means learns, conclusively, that vampires are real, maybe along with the reason nobody talks about them or knows they exist. Then boom the next chapter stars a vampire.
You cannot make the new species look like a deus ex machina just to get your main characters out of a jam or solve some other problem. If they struggle against a bad guy, and your solution is "There are vampires, the vampire kills the bad guy, problem solved", that is not satisfying.
In other words, the vampire should not simplify anything for your main characters, it should complicate things for them. And unless you are writing a farce, don't make the vampire just another cardboard villain for them to put a stake in.
If you introduce a vampire character out of the blue, it is best if the vampire has a new problem, and the vampire's attempts to solve their new problem causes more new problems for the Main Character (or Main Crew).
So three parts: First throw a spotlight on why nobody in the world ever talks about vampires, Second give the vampire a new problem (an inciting incident that puts them on a new path to intersect the MC), and third, make sure you have complicated the story, do not let this introduction solve any problems for the MC.
Later (as EDL noted in Harry Potter) the vampire may indeed be instrumental. Later, in solving some problem for the MC, perhaps they cooperate, or perhaps the help to the MC is a side effect of the vampire solving its own problem (i.e. that is how the vampire's arc is concluded).
But not immediately; in its first chapter the Vampire must contribute to the troubles of other POV characters, and should have a real and new problem of its own to overcome, some motivation for changing its routine. It would be great if you can tie the vampire's new problem back to something caused by the POV characters; then the Vamp coming out will look like an unintended consequence of the Main Crew's actions.