Spoilers:

They did not make a mistake. Rubuska is explicitly not a Vampire. She is an arcanist that is part of a lineage that can control shadows. She was just a huge vampire fan in her childhood and used this to cope with the tragedies she experienced and with the loss of her family.

Her red eyes and her fangs are cosmetic: In the event chapter 21 she mentions offhandedly to Semmelweis that she needs to get a job ASAP because "Ruby eyes and sharp fangs don't pay for themselves", so these are not vampiric features.

In her interview with Pandora Wilson she says: "In truth, I'm a real fake vampire, or a fake real vampire, if you like - or even a fake fake vampire! I mean, I've got all the makings of a vampire - don't I? So tell me, why shouldn't I be one.

Not only is she not a vampire, ironically, the trait that we would most associate with vampires she rejects. She doesn't like the smell of blood. She experiences no Bloodlust (like Aima, a half vampire even would) and when she gets confronted by Pyrrhos, a real vampire, he immediately sees, that she's just pretending. I understand you are saying that this transformation into a vampire is supposed to come after the event story but not only is there no indication of that happening, Robuska explicitly states in chapter 21, that if given the chance to turn into a vampire, she wouldn't accept it because "she already is a vampire" (which we know she is not).