But see, that's what gets me. Why even bring it up? It's pretty significant that Remi went to Flan's room and neither were seen again, and mentioning that in-story makes the reader curious as to what happened. If it's not mentioned, the reader can just assume they're off stage somewhere and not worry about it.
In any other story the vampires vanishing would be as much an incident as them appearing in the first place, don't you think? Certainly not something that would be greeted with "welp, these things happen" by those loyal to them.
Is possible that Meiling and Sakuya know perfectly what happened to them, and well, the reader shouldn't worry about it since the focus is Meiling and Sakuya's relationship.