Formula to look up a corresponding field from a related table?

The report won't work because I need the data to be stored in the vehicle record. Additionally, I cannot change the key field, because it would affect my integration with another system. I'm thinking perhaps making the vehicle record table a child table to a table where the last 6 of the VIN is the key field. I'm still trying to play this through in my head, so if you can contribute any pros or cons, that would be great!