The easiest way: Use the contact form and write your relocation/cancellation request together with a scan of your deregistration form. Please note that this should be done in German language as German is sole contractual language - requests in other languages, especially in English, might be discarded/ignored.
Other way would be to send a letter - same notes apply.
From the moment Vodafone receives your deregistration notice & cancellation request, you will have to pay for additioal 3 months in accordance to the telecommunications law - as you haven't noticed Vodafone in time.
And note that you have to return any rental device in time - otherwise you'll have to pay a fine for not returning the devices.
