Get your neighbour to report the fault and agree with you the access arrangements for the VM tech who will come to repair and replace. Don't get involved with VM yourself. You are right they won't deal with you. Only the account holder. But you can be helpful as to access.
I'm a Very Insightful Person, I'm here to share knowledge, I don't work for Virgin Media. Learn more
Have I helped? Click Mark as Helpful Answer or use Kudos to say thanks
The OP referred to the cable on his property which serves his neighbour, which often means a shared cable even if one of them is no longer a customer. And if it is a shared cable it is worth dealing with it.