Unless they left a big U on the side, I think you are making some big assumptions. BYU is largely disliked by every other school in Utah.
Oh when I drove my car covered in Ute regalia to the "54-10" game some nice BYU fan wrote I love BYU on my back window and other BYU stuff on the car. Granted they did not get inside but I certainly did not consider it "vandalism" I thought it was funny and took pictures and sent them to my friends.
Now maybe they (whoever they is) really made a mess of your car, if so that's too bad and kinda sucks. But if I drive my Ute Car to Provo, anything short of permanent damage is expected. It is part of the fun.