Is the MX64 set up for passthrough? It sounds as if you have an IP config mismatch between your MX and your Vonage box. Are IP addresses manually set, or can you do DHCP? If you are doing DHCP, which is the server (cable modem, MX, or switch)?
I would set it up this way:
Cable modem to MX64
If MX is set for passthrough, Cable modem doing DHCP, excluding a zone for manual IP which will be used for MX, switch, APs, and maybe Vonage box. If MX is set as router (NAT), have it do DHCP and turn DHCP off in cable modem.
MX to Switch.
Hang everything off the switch.
Alternatively, you could hang the Vonage box on the MX, but I'm not sure what you're trying to accomplish by this other than saving a switch port. Make sure your Vonage box is on the same subnet as the MX on the "inside" of your NAT, or just set it as a DHCP client.
I don't know a way to do a factory reset from the cloud management portal. I suspect that can only be done at the device, physically, or through your subnet by logging directly onto it (use computer on same subnet and navigate to the MX's IP address using a browser).
I hope this is helpful. Let me know if I need to clarify something or have made an incorrect assumption.