Forum staff only process cancellations on an exceptional basis, so although they have done this in the past, the general response is to say they can't deal with account and contract changes through the forum. That's company policy, so if you get a response that they can't help then don't take it out on them, as they may still be able to do something to assist as per my last paragraph below.
If you want to leave without penalty and forum staff aren't going to help, you'll likely need to raise a formal complaint with VM, seeking that as the resolution (or them to resolve the problems you see within say two weeks, since they've already had time to address the problem). That complaint will likely be fobbed off, if that happens you reply to the resolution letter, rejecting their resolution and asking for a deadlock letter and with the deadlock letter you ask Ombudsman Services to investigate and resolve. If you haven't managed to get a BQM setup, then I suspect you won't have very compelling evidence, and that may be a problem.
However, if you've got (or shortly get) a price rise notification from VM under which you can terminate without penalty, that will be a much easier and quicker way of leaving, rather than trying to use a complaint about poor connection quality. If you haven't already heard, forum staff may be able to advise whether your account is in scope for this year's price rise.
