Eeeekkk! No, you do not cancel the direct debit. Bad things will happen or you, but VM won't care, it's just a process for them. As far as I can see there's no option for postal cancellation with VM mobile contracts, so you'll have to endure the company's miserable and incompetent telephone customer service.
Hi KarenH1210, thanks for posting and welcome to our community.
Sorry to hear that you're wishing to end your contracts and sorry you've been unable to get through to the right team. Here is a direct number to our mobile customer relations team: 08001831150. I believe they are closed for the evening now, sorry.