The only reasons I can think of, to do that, would be if you are using consumer Google Voice, which isn't supported on the VVX phones, or if your SIP provider doesn't allow multiple registrations. Otherwise, just set up your SIP credentials on the VVX phone itself.
And yes, there are a lot of very long threads dating back years on the problems people faced trying to do what you described. I would not call it a "beginner" task to set up.