Sadly, there are no methods for resolving this that the customer can use.
Yours is a regular story of failure and bungling incompetence which, unfortunately, is reported on a near-daily basis on here.
The way VM has set up its sub-contracting arrangements for cable installations seems to be that, once the job has been allocated to the sub-contractor, VM has no direct control or influence on how/when/if the job is carried out. When you call VM for information, the call handler is doing nothing more than reading the date showing on the screen in front of them. They have obviously been told to reassure the customer the date is valid as it is often quoted on here that the call handler will 'guarantee' the next date, only for the customer to find nothing happens on said date.
I don't think any reliable methods to speed up or change the process have been posted on here. All you can do is wait and keep very detailed records of all VM's failures, delays and false promises as you will be entitled to compensation for the delay.
https://www.ofcom.org.uk/phones-telecoms-and-internet/advice-for-consumers/costs-and-billing/automat...
Keeping evidence of the 'date initially confirmed in writing' when VM said they would 'activate' your service is important as that would be when any compensation starts from (para 9 below)
https://www.ofcom.org.uk/__data/assets/pdf_file/0026/216962/Industry-Code-of-Practice-for-Automatic-...
Keep 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.
