"I'm still young" ooh... ?how does aging work for vampires then? this raises a lot of question like how old is she and how many years she needs to be an adult vampire, and then I guess she isn't inmortal?
It may be the case where one stops aging when they turn into a vampire, and after that they gain power over time. Since it was mentioned that Miyoko was already centuries old, I'm guessing that she has completely stopped aging, or at least ages extremely slowly.
Also, that other girl is definitely a vampire hunter. I'm betting that the next few chapters or so will be about Saki defending Miyoko from her, or about Saki trying to keep her from telling Saki's mother that she couldn't kill Miyoko.