If you really want to use your own kit make sure that you can change the MAC addresses on it, then change its' HFC MAC address to match that of your current VM supplied cable modem and it should work fine.
As previously mentioned against T+Cs but perfectly doable.
The wireless router ingress sounds more like unterminated cable points sucking up the wireless signal. An eMTA or similar combined cable modem and home gateway will be absolutely fine to use on the VM network, chances are it'll be a better quality piece of kit than the supplied modem.
Agan just be aware that VM don't like you supplying your own equipment
