That is the answer to your question. And the answer you alread got, but detailed. You don't need additional models for VATSIM, they are already installed if you are using flightgear.
FG comes with a separate catalogue of models, used for AI Traffic, multiplayer models and apparently for SWIFT/Vatsim multiplayer sessions. If a model is missing, a reasonable alternative in size and configuration will be chosen.
You should maybe try to formulate better questions, however. If you are having problems with the AI models, you need to explain what you are trying to do, what version and OS you have, where you downloaded the plane/content/tool you have problems with from... We cannot guess from one line what the real problem might be. Flightgear is a highly complex system and errors can creep in in a million ways. If you get questions instead of answers, it is because we are trying to figure out what's going wring - as long as you get responses in the forum, someone is looking into it and thinking about it.
And by the way - welcome to Flightgear and the forum