At the moment VM's network still uses EE, and would continue to do so whatever until Autumn 2021. So nothing's happened yet, and nothing is likely to happen soon.
VM signed a deal last November to transfer VM's mobile service from EE's network to Vodafone, a transfer that would probably happen next year as the deal with EE expires in Nov 2021. On paper that's a five year deal with Vodafone Nov 2021 to Nov 2026. Meanwhile, VM were negotiating to merge with O2. It is certain that the contract between VM and Vodafone has provisions for termination of the deal, but what they are, and any penalties will be confidential.
So, what happens now? Nobody knows, and certainly nothing until the merger gets competition authority approval probably early 2021. At that point, VM or Vodafone may be able to extract themselves from that deal, and agree that VM mobile customers will instead move to O2. Or, if there's no opt out clause that VM can use, and Vodafone dig their heels in, then VM will have to use Vodafone for the next five years, and then transfer to the merged VM/O2 network.
On the assumption that Vodafone don't have to let VM welch on the deal without penalty, I'd expect VM to be looking at ways to buy themselves out of the Vodafone deal, and Vodafone would be quite happy with that so long as there's a big cheque attached. In the grand scheme of a merger with costs of the order of half to one billion quid, paying many tens of millions of pounds to extract themselves from a deal that doesn't now suit them would be small change.
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