@SHM-- You are both right and you're wrong. Although VATSIM, itself, is free, it is an addon that only works with proprietary flight simulators.
Users need custom plugins (such as SquawkBox, FSInn, vPilot or XSquawkBox) for flight simulators (such as Microsoft Flight Simulator, Prepar3D or X-Plane) to connect as pilots, or radar simulation applications Advanced Simulated Radar Client (ASRC), Virtual Radar Client (VRC), EuroScope, vSTARS, or vERAM[21] to connect as air traffic controllers. VATSIM-specific applications are available for free and provided by the network itself.
Source:
https://en.wikipedia.org/wiki/Virtual_Air_Traffic_Simulation_Network#SoftwareFlightGear is both free and Open Source.
And of particular note, in contrast to FlightGear, VATSIM is not Open Source software. It is completely proprietary and run on proprietary servers. This is not to say that it is not a high quality software and simulation package. It is a very impressive system. However, it not being Open Source places it in a category that makes it impossible to ever be integrated with FlightGear. Therefore, from our perspective, such software, no matter how sophisticated it might be, just can't be used as we develop FlightGear for the coming generation. We have to look to Open Source initiatives to build the next generation of Air Traffic Control systems. It can't be based on VATSIM, so naturally, its models for training Air Traffic Controllers have to look different and it will be essential that FlightGear players train for the FG ATC system, not VATSIM, when it comes along.