Rin's expression makes it apparent that she's gone on a bit of an emotional journey although Vergil couldn't exactly say what said journey is or why it was sparked by Vergil's question in return. Before he can begin to try and parse it out for himself, she's already quickly attempting to dismiss the topic of conversation altogether.

"You know Mizu as well then," he concludes aloud as it seems quite unlikely to Vergil that there are two people among the Star Children that have taken such an intense interest in London. At the very least, Mizu would have likely mentioned that other person by now or Vergil would have seen them the handful of times he's joined the other swordsman in the library. Still, it is not as though it's a closely guarded secret that Mizu has such an interest. Mizu does not work under the cover of night nor does he exactly hesitate from asking others for information on the chance they might possess useful knowledge. Studying her carefully for a moment, he asks, "Did he ask you not to involve yourself?"

Mizu's fierce independence aside, Vergil doesn't think Mizu would want to involve someone as young as Rin looks to be in such business. Especially when not when she seems so prone to reactions as the one she just had. It would add an unnecessary complication to what is otherwise straightforward business.