Ben, I hope that you don’t mind me mentioning this but you do not seem to be very familiar with the units and how they work together. I am assuming that you have not obtained these from an authorised dealer who should have explained all of this to you. Which leads me to wonder if you have the user manuals? If not let me know and I will send you links to them.
The RS232 cable does not carry any music information. It is concerned only with control data i.e. how the boxes in a dCS system talk to each other and Mosaic. On the other hand the ethernet cable connections carry the music data itself and its related information.
So a Vivaldi system has both ethernet ( or S/Pdif or USB ) connections and RS232.