Simple enough: a lot of the time I want people to know I'm online but not where I am.

This is easily possible via ProtocolLib. Use packet 201, PLAYER_INFO.

There's also TabAPI now, which hooks into ProtocolLib and allows for easier editing.
http://dev.bukkit.org/server-mods/tabapi/

May I ask why this is not possible? VanishNoPacket can already decide whether to remove the user based on the "vanish.hooks.essentials.hide" permission, why not also only do this when we use "/vanish fakequit". It's already possible to allow/disallow this feature per-user, so why not per-command?

So it would work like this:
/vanish = Don't hide in Essentials
/vanish fakequit = Hide in essentials

Essentials doesn't control the tab player list.

I believe this will be impossible as of 1.8

Currently not possible under vanish api but will look into it