My thought is the van used to belong to a Porno star mom who's child has reached the age where he/she is embarrased by Mom's occupation. In a fit of rage she retaliates at the ungrateful child. "Yeah I suck dicks in pornos, it's what I had to do to support this family. Everything we have we owe to suckin' dicks. Suckin' Dicks bought this van that's driving you to the mall." The plate holder was given the next Christmas by a child who now understands how things work and is proud of his Mothers devotion to her craft. Or maybe, it's a single mom having a talk about the facts of life with her teenaged daughter. "You might find it distasteful at first, but you'll see that men are very grateful to girls that will go down. In fact, suckin' dicks bought this van."
I think the plate holder was put on by someone who was proud of the fact they were such successful blowjob artist. They would probably be upset they left it behind when they sold the van if they didn't now have a "buttfuckin' bought this vette" plate holder.
~Paul