A few times today, a player without the ability to see vanished players saw a vanished player (myself). I checked my vanished status, and I was (allegedly) vanished. Another staff member came to the location, was not seen, until he teleported again. After the teleport it seems that he was noticed.

Do you have a mob-disguise plugin or similar? Coinciding the 'on' statuses of such plugins and VNP can be unpredictable, messing up either or both, even after toggling one off.

It was actually the Spout plugin, removing that fixed the issue. We noticed console spam from spout lined up with becoming visible.