Simply either use the contract form (you don't need to be logged in) and send your cancellation request -in GERMAN language, as it is the sole contractual language- including a scan of your deregistration letter.
Include your customer number and -if known- the phone number of the GigaCube. There is no need for any kind of registration of your contract in order to cancel it.
But I am not completely sure if a premature cancellation of your contract is possible - usually this type of premature cancellation is only possible for landline contracts and not for mobile contracts as mobile contracts may be used freely within Germany (and for most even within the world - of course, with roaming charges being applied). In so far it may happen that you still may have to pay until the end of the contract...
In case it's a GigaCube Flex contract, this is no further problem as this contract will cause no fees in case it is not used at all. But for the regular GigaCube contracts, it may happen that you have to pay the outstanding monthly fees until the end. At least there will be a 3 months charge for the next 3 months after the cancellation request is received by Vodafone if the request is processed like the landline contracts.
