OTS was introduced on 12th September 2024, however it still isn't working properly.
One of the problems is that the switch only covers landline and broadband. It doesn't cover TV services and the customer needs to contact the losing provider (Sky in this instance if the OP also has a TV service with Sky) to cancel the TV service. See ofcom.org.uk/phones-and-broadband/switching-provider/switching-broadband-provider "
If you use a bundle of services with a provider, for example home phone and broadband, you will usually follow the process linked above. However, you may need to notify your old provider to cancel other services you are also switching as part of a bundle - particularly if the switch involves cancelling a TV service. Your old provider will give you information on what your options are regarding these services in the switching information they send to you
The article linked below which shows that daily the daily switch match success rate is currently hovering around 67% – 68%
/problems-remain-as-uk-broadband-isps-mark-first-year-of-one-touch-switching.html
The gaining provider (in this case VM) are responsible for the landline and broadband switch and should be able to see what went wrong. VM should either re-send the "switch complete" message or trigger a manual disconnection with Sky.