Bit late to this thread as I left VM a year ago now. But before I left I did the threaten to leave (bluffing as I had no other real alternative to switch to), they did virtually nothing. I think they reduced the cost a tad, but didn't increase speed or give extra TV bundles etc. I even went down the road of cancelling, hoping they would offer something else but nothing. I had to call back to cancel the cancellation.
I then decided to ditch 'live' TV altogether, and the phone line which I never used but stupidly was cheaper to have in the bundle than just getting TV and internet.
I signed up with EE internet before even calling VM as I was sick of the price rises and crap offers, despite being a customer for over 10 years.
THEN they offered some better deals, but I said it was too late. They transferred me to two further people for more deals and wouldn't take 'I'm leaving and have already signed to a new provider' as an answer! Eventually they did cancel!
I don't know how some people get the deals they have here (or in the VM thread), I guess just who you get connected to on the phone I guess.