the VIN is absolutely NOT needed to lookup most parts but it is the best way to ensure that parts are applicable to a certain model. If a dealership is giving you flak on a part that you KNOW is transferrable to your vehicle, tell them you know what their **** looks like and they don't need a VIN:
You just have to populate all the fields manually if you don't supply a VIN. When you click on the <?> next to Model Code, it gives you a list of vehicles... G11 applies to 02+ Impreza. After that, you fill in the remaining boxes that have chooser buttons and you can ignore the ones that are just blank fields.