Hi JohnRob.
Using cat5 cable is a cheaper option and grounding the remaining wires in the cable will help.
I'm not electronics savvy, but the Vin question can be an issue. I believe that in this case another question would be the protection of the Arduino port.
If I use the optocoupler, I believe I need to provide another power source for the switch. Could using 24V be a better choice due to the distance?