Usual advice on here has been to try phoning first thing in the morning, when the lines first open, and go via the 'thinking of leaving us' options (or nearest equivalent).
Plenty of recent topics on here do, however, speak of the difficulty in making contact with VM and obtaining a reasonable and realistic renewal price.
Do your research into alternative prices and packages and have that info to hand when you speak to VM. If VM does not come up with a suitable renewal price/package put in your cancellation.
After cancelling you might get a call back with a better offer but there is no guarantee that will happen so, if you do cancel, you should be willing to see it through and actually leave.
If you return to VM later on in the future, you will be a 'new' customer once more and will get new customer pricing again.