Your choice on whether to cancel VM or not, but I advise you don't cancel or recall the direct debit. Nobody at VM will see or notice that, but VM's systems will, and they'll process it as a late and then defaulted payment. They'll add late payment charges, and pass it on to a scumbag debt collection agency who will harass you. Worse, it'll put skidmarks all over your credit history, with the potential to cause all manner of problems for the next six years. Credit history is visible to, and used by mobile phone companies, ISPs, credit card companies, energy suppliers, water companies, mortgage lenders, landlords, car finance providers etc etc, even large employers will routinely check applicants credit history.
No matter what the problem, it's always best to leave a DD in place and keep the payments flowing, and separate to that to use VM's complaints process in regard of the underlying problem, and when that produces a fob-off response, then take the matter to Ombudsman Services.
Some people are under the illusion that withholding a monthly payment of £30-100 will make a human sit up and take notice at VM Towers, to recognise some underlying problem, and proactively sort things out. I'm not sure why they think this. VM's residential business bills customers around £300m every month. In this age of computers and "business processes", people do nothing at VM other than answer phones. All payments are automated, non-payments are fed into the fully automated bad debt processes, and if you fail to pay then at the end of the month the only human intervention at VM will be a junior finance analyst updating the Powerpoint slides for finance leadership team, and somewhere in 30 pages of numbers that'll include the total value of overdue payments across all of VM's customers. If you recall your direct debit, that'll increase the number, but it'll be a tiny proportion because the overdue customer payments will likely be in the low million pounds per month range. Nobody will see, nobody will care, nobody will do anything (other than the debt collection agency, they're pretty hot on chasing down people who choose not to pay). Late payment letters from VM will likewise be automated - if they bother to send them at all - as will be the disconnection process if you still don't pay.
In some situations you can be legally entitled to use non-delivery of a service as grounds for terminating a contract, for withholding or clawing back payments. But that won't stop VM's systems from processing your account like a bad door in Monsters Inc, hence my advice to keep paying and use other routes to address the problem. If you're due money back, Ombudsman Services will make sure you get it.