Hello Abdalrhman,
Yes, you can fly in VATSIM using FSX, but unfortunately, the default sceneries in FSX are outdated. I would suggest downloading scenery from third-party websites, as there are plenty of options available. If you are looking to buy scenery, you can check simMarket, and for freeware, I used to check on freewarescenery.com. As far as I can remember, most of P3D v4/v3 scenery works for FSX too.
Hope this helps!