Seems running the command sim_vehicle.py --no-mavproxy still results in mavproxy being run automatically.
The docs say with the --no-mavproxy option, and alternative GCS can be used:
" Using a different GCS instead of MAVProxy¶
Start sim_vehicle
without starting MAVProxy using the --no-mavproxy
option. SITL will be listening for any GGS station to connect via TCP port 5760."