I believe the Essentials project is being maintained by Spigot staff? Or am I confused?:
https://hub.spigotmc.org/jenkins/job/Spigot-Essentials/

In any case, the issue I would like to report is related to vanish handling with Essentials and Premium Vanish plugins.

It seems players vanished with PremiumVanish, can be identified through the Essentials /seen command, allthough seemingly, a propper hook has been coded, due to the fact that vanished players are shown as offline since "now" or "1 second" (Which is quite suspicious when the result is the same after executing the seen command a few times) Players can use this exploit in order to detect invisible PremiumVanish players.

I've already reported this to Premium Vanish, but the corresponding developer seems to find this as an issue on Esential's side:
http://i.imgur.com/zCmdDIy.png

Please read from here, if any more details are required, or ask: https://www.spigotmc.org/threads/premiumvanish-stay-hidden-bungee-support-paid.102401/page-23#post-1635192