I'm not sure, but I saw this somewhere else, how you could start typing someone's name, press the tab button, and ALL online players would show up who had what you typed in your name (Even people using /v). I could not get anyone that had investigator to do /v on the server, so I'm not sure if that is actually a problem on ECC. I just was checking to make sure it wasn't here. Thanks
I don't think this is a bug, as the tab auto-complete is a part of Minecraft on purpose.. My guess is that the /vanish devs can't prevent it from happening, or it never came to mind since it doesn't happen much.