You will get no reliable answers from VM, unfortunately. The cabling work is sub-contracted and, once the job is allocated, it seems the task is left entirely to the sub-contractor. The dates VM keeps handing out, when you phone, are just automatically generated on the screen in front of the call handler and typically have no relationship with the reality of when an installation may happen.
Splitting a cable from a neighbour is a temporary measure to get you working but often, when this happens, the split cable ends up left like that indefinitely. If you are having a new installation, VM should provide you with your own cable (particularly if you are on the highest tier speed).
You should start keeping a detailed record of events (dates, times of all calls, messages, texts etc. along with any missed appointments and failed appointments where someone turns up but no work was carried out). Keep the record in a timeline format with links from the timeline to each piece of recorded evidence. This will make it easy for you in the future to put forward a case to arbitration for the correct compensation, should that be necessary.
https://www.virginmedia.com/help/billing-and-payments/automatic-compensation