Transmitting VCC and GND signals for use with RS-485
Properly regulated and filtered (decoupled) supply rails will have practically no a.c. content. Therefore the will not radiate into your communications lines and interfere with communications.
Decent decoupling capacitance at both the supply source and the load will keep the supply impedance very low. If the RS485 communications lines are properly terminated and with your 115,200 bps bit rate, the effect of the communications lines on the supply rails can go from small to practically nothing.
To compensate for voltage drops caused by the load current and the cable resistance, you can provide a higher supply voltage at the source cable end and regulate it down to 5 V at the load cable end.
If the load current was low enough, you could have used a linear regulator and the losses may be insignificant to your system. Example figures (based on no application data) might be supplying 9 V down the cable to power a 5 V LDO regulator in your sensor.
However, with the 500 mA maximum load current, it is worth using a switching step-down regulator in the sensor. An advantage of that is the supply cable current will drop as the source-end voltage is increased. The disadvantage is more complexity that wouldn't be worth it for a low current load.
I can't find max. current data for CAT-2 cable but CAT-5 cable is in the order of 577 mA. So you could use a 12 V source cable-end supply and a load cable-end switching regulator to drop 12 V to 5 V. If the load-end regulator was only 80% efficient, it'd draw 260 mA from 12 V supply to deliver 500 mA to the load. This ignores the losses in the cable, which is your further consideration when you select the actual cable. But your actual currents are likely to be much lower, from your question, and more efficient regulators are readily available.