I believe that you are able to add your own notes to add to any entries that you dispute, which would allow you to explain that it was fully paid and that you have asked VM to remove it, but have thus far been ignored.
If frontline staff give you the run around, i'd write to the CEO of whichever credit reference agency is involved and VM.
The principles of reciprocy may also be helpful, more details here:
http://www.scoronline.co.uk/principles