Place it at the end of a short anime reaction edit, right after the confession it answers, then follow it with a reaction sound to let the moment settle before the next scene. An anime gasp works well just before it, setting up the reveal without stealing its moment.
關於此聲音
This is a very short spoken line, just over a second long, in which a voice plainly states that Vash likes feet. The delivery is blunt and matter-of-fact, which makes the odd preference land as a joke rather than a confession. It feels awkward, silly and a little uncomfortable, the kind of line that gets a laugh because it is so specific and so casually dropped.