Write to VM, at section N of the terms and conditions. And post it recorded delivery.
Since you have to give thirty days notice you'll have to pay for a few weeks when you don't need the service, but accept that, you agreed that contract. Because there may be a delay in postal processing you may yet get charged another full month (noting the thirty days notice anyway), but it should be backdated and any overpayment refunded. Leave any direct debit in place - far better to dispute a charge afterwards than have a big brown smear for six years on your credit history due to a defaulted payment.
Any problems, come back here and we'll advise further steps.
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