Bukkit

Do you have a plugin that adds Vanish?

If you have Essentials Installed

Don't worry you won't need to remove it. But in the config.yml where it has disabled commands. Add 'vanish' and 'v' without quoets. For example:

Disabled Commands:
    -vanish
    -v

And that is all

If you have any other plugin that adds vanish

See if u can disable the command for that plugin in the config or ask the developer to see if he/she can add support for disabling commands.