If you have a vanished player on the server, and all the unvanished players try to sleep, they can't until the vanished player does also (or leaves the server). Clever players can thus figure out there is a vanished player on the server.

This should not happen, as VNP uses a Bukkit method to mark the player as ignored. I'll look into this.

From a review of the code, when a player vanishes their state is set to sleeping ignored.