Very mixed results - some people get excellent retention deals, some get poor or no deal. Some get offered a poor deal, reject it and cancel, and then when they give cancellation notice, VM make an outbound call to offer a good price during the 30 days notice. Some get offered a good deal, but then find a different one has been put in place, and have to fight to get the agreed deal honoured.
Your best hope is to be fully aware of the competitor offers, and be prepared to change ISP, then phone up VM, navigate to "thinking of leaving", and see how you get on. Don't mention loyalty or fairness, those don't count - focus on the fact that you see VM's new pricing as poor value, and unless they can do a lot better you'll leave.
Some notes here to adapt to your circumstances.