If you cancel your contract at the "usual" end of the minimum contractual term, there is no verification or any other documents needed. But it'll run then until the end of the minimum contractual term.
If you want to cancel earlier, you'll need to provide proof that you left Germany and/or moved to a location where your contract cannot be served as agreed for. In that case, you'll have to provide official proof (e.g. letter of deregistration / letter of movement by officials like the town hall), flight tickets are NOT enough! You may ask for that termination in advance to leaving Germany, but your letter must state where you move -and- you'll need to send the official documents later on after you received those documents - your contract will in any case be terminated by the date you relocate your relocation request is been received by Vodafone (the later one counts) -and- you'll have that 3 months penalty from the date you relocate or the date your relocation request is received by Vodafone.
How that process works:
Send a letter in German(!) language (as this is the sole contractual language) to Vodafone with your relocation request (new address + date) - and, in case you already have, with the official proof - and that you want to cancel the contract immediately due to relocation. In case you didn't provide proof in your first letter, send the official proof as followup as soon as you get it. And of course, provide a new address for Vodafone to reach you in case of questions.
In that case, your contract may be ended by the date you move or that your request is received by Vodafone - the later one counts (plus the 3 months notice period).
