Honestly, I'd say just cancel and go back to Sky, or alternatively, you go back to Sky but don't bother cancelling and think about the compensation you are owed. I assume that VM gave you an installation date, and hence you are due compensation from that date onwards, irrespective if you cancel or not.
The options which Roger_Gooner (above) posited are perfectly logical and would be the way to go. Except VM's systems simply won't allow for such a thing, it won't be in their list of 'processes'.
Far too late now, but honestly, your best option would have been to cut the cable to your neighbours' eighteen months ago, since, as you have rightly observed, events such as the one you find yourself in are well out of VM's comfort zone, and hence they ignore it and hope it goes away - which it did, well for 18 months!
Do you want to force the issue? How well do you get on with your neighbours? Because the only way anything will happen is if you now (accidentally) sever the cable between the box on your wall and your neighbours house. When they complain to VM and a tech turns up to fix it, you absolutely deny them access to your property. It'll really, really annoy your neighbour, but probably the only way any action will happen.
