I know that at Subaru, we run 3 different systems that don't cordinate/communicate with eachother. If you have a part number or a paint code, the code may be supercessed or different in our invoicing system. With a VIN, I can go into our parts program, pull up your paint code, match it with Subaru or America (to see if they haven't changed/updated numbers) then plug the most updated number into our invoicing system.
Thats one of the reasons that it may seem like we are taking an eternity to find a simple item. We are just trying to PREVENT a miscommunication/wrong part. With the correct last 8 characters of the VIN, its just one less variable that could cause a problem.
Nothing is guarnteed without the VIN.