The question is not when you in reality moved but what the deregistration letter says. As this deregistration letter is the only official proof that you "really" left Germany...
If the deregistration letter says that you've left in December, then all is ok and your cancellation will be processed for March. Because of the current situation, it may take some time until your letter is processed - it may therefor happen that you'll get additional invoices until this processing has finished. In any way, you'll be reimbursed any additional money that has been charged on top of the fees that you'd have to pay usually within the next billing cycle.
But if your deregistration letter says that you've left Germany in March, I'm afraid that you'll have to pay until June - as it's in any case 3 months from the day you officially left Germany. It was your obligation by law to deregister in time - if you did not do so, it's (sadly) your own fault that leads to these additional charges. Vodafone only uses the official deregistration letter as source for calculating and readjusting the premature cancellation date in accordance with the telecommunications law.
Just as addition:
I hope you've already sent a copy of your deregistration letter to Vodafone - if you haven't done until now, your contract cannot be cancelled as you have to hand in that copy as proof! And only after this proof has arrived at Vodafone, your cancellation can be re-adjusted accordingly - otherwise the contract will run until its regular end of the minimum contractual term.
