I would find out what is causing your slow speed before cancelling. as not all causes of slow speed are VM's fault.
How are you testing the speed, wired or wirelessly?
If wireless, I would test the speed wired directly to the hub during the day, and again during evening peak period. Wireless is just too unreliable to test the actual connection speed accurately as it is affected by a myriad of external factors outside any ISP's control like Range, Client Hardware, Interference and the topography of your home.
If you are getting headline wired speed all the time then the fault will lie on the wireless side.
If the wired speed is poor all the time, then it looks like a technical fault which can usually be fixed fairly quickly.
However, if the wired speed is good during the day, and slow during evening peak, then it looks like a contention issue. These problems usually take a long time to fix. If this is the case you can contact VM and ask them to release you from your contract without Early Disconnection Fees, as VM will not be able to fix the problem in a timely manner. If they refuse, make them aware you will be putting in a formal complaint to CISAS.