We updated to the new 1.7.2 bukkit version today, and as a result nobody can use vanish no packet, as minecraft itself now has a vanish function (which doesn't do anything, lol). Whenever I type /vanish or /v, it says /vanish, /v on:off (but doing /v on or /v off doesn't do anything).

Could we get an alternate command to use for this?

Minecraft does not have a /vanish command. Please pastebin your server startup.