The C box itself is not worth much (less than $100) to anyone except a Vantage homeowner that needs to replace their unit.
A Vantage system, even Qlink IMO should be head over heels better than Insteon.
An upgrade to Infusion (which is much powerful than Qlink) is only necessary if you want to add av, cameras, touchscreens to your system.
The upgrade to Infusion is in a new controller and programming, probably nothing else.
How many Vantage switches are there in your home? Is the system wired, RF, or a mix?
If you are techie you may be able to get a copy of the Q link program and modify your existing program youself.