Staff on my server do not see anything when non-vanished players quit/DC. If they're vanished we see "Soandso has quit vanished" but I feel as if people who can see that announce should also see a quit message if they're visible.

You gave them vanish.silentquit

I checked my permissions, and they do NOT have "vanish.silentquit" but I am still not seeing any notification for non-vanished players leaving the server.

Then it's unrelated to vanishnopacket.