VM's T&Cs refer to this in part Q. Visiting your home Section 1.1
https://prod.ctassets.virginmedia.com/uploads/Terms_and_Conditions_TV_Fibre_Phone_from_4_February_2024_35cc93ecec.pdf
If your box was on the boundary between the two properties, it might be a 'reasonable' interpretation to loop a connection to the next door neighbour.
Running a cable to the neighbour across your property is not, however, a 'reasonable' interpretation IMO. Being a VM customer does not sign you up to be a cable distribution point for the surrounding properties.
As jpeg1 has already advised, this is likely just be an attempt for the contractor to save time/money/effort in providing the neighbour with their own connection.
We have seen past examples on here where an existing customer has allowed this as a goodwill temporary measure to get a neighbour's connection working with the promise of a new cable for the neighbour in the near future. Once the split connection is established, and both properties are connected, there is no motivation for VM to ever replace the split connection.