I am not familiar with VATSIM, but as far as X-Plane settings go, I would recommend you check the "other aircraft" tab found under Aircraft menu -> Aircraft & Situations. If the number of aircraft is greater than 1 (for yourself), you can see what aircraft are being loaded for the AI aircraft. You can randomize them or pick specific ones by using the big check box next to the line. Double check there that AI aircraft aren't all set to the same airliner/livery.