Vanished players can be found whenever a player presses space and then auto completes and as a result gets a list of all players including vanished ones.

nope, https://github.com/Bukkit/Bukkit/blob/master/src/main/java/org/bukkit/command/defaults/ListCommand.java#L29

I think this is related: When using bukkit buildin command "/list" vanished players are displayed in the list

Ah - I See.
We uses Essentials.
It must be Essentials' list command our players trigger.
I've removed the permission for Essentials list for now.
I'll try to add bukkit's permission for the list command.
Maybe that'll solve the problem and still let our players use the list command.

Thanks :-)

You should be able to use Essentials as long as you enable the hook. In theory it would remove you from the list.

I'd just like to point out that this bug still exists in current versions.

This shouldn't be happening. You do mean pressing tab while chatting right? https://github.com/Bukkit/CraftBukkit/blob/4d832c39f320f7745a210a9d9a0accc665bd49dd/src/main/java/org/bukkit/craftbukkit/CraftServer.java#L1587-L1591

As far as I know people are able to complete my username if I am online and vanished by typing "god" and then pressing tab for example.

I am using MCPC-Plus build 80 http://ci.md-5.net/job/MCPC-Plus/80/
Vanish v3.19 http://dev.bukkit.org/bukkit-plugins/vanish/files/84-v3-19/

But does it happen on CraftBukkit?

Silly me, this is not an issue on normal CraftBukkit.
I will create an issue on MCPC+ and see what they say.

Thanks for checking, @godgodgodgo :)

For anyone following this but not the MCPC+/Cauldron issue, it's now fixed!