Leave all the stroy and plot aside, there's only one thing I need to explain:
How did the vampire see themselves in the mirror ?
I don't know, how did the vampire see themselves in the mirror?
What, it's not a riddle?
Well, more seriously, vampires in this are physical beings, so of course you can see them in mirrors. The whole mirror thing doesn't really date any further back than Dracula, and seems to be a combo of vampires not having souls (mirrors reflect souls, y'know), and mirrors of the day being made with silver, which was a ward against various sorts of supernatural beastie. Presumably, since we were already told a lot of vampire stuff like being out in the day (and probably running water too) are just folklore, then that doesn't apply either.