I'm trying not to bother you any more but you did ask me to give you a progress update. I am stumped. I have working on this the better part of the day and I haven't made much progress. I placed the following code at the end of your code:

SELECT VehicleDetail.SerialNumber, VehicleDetail.ProgYear, VehicleDetail.ModelName, VehicleDetail.Region, VehicleDetail.MFG

FROM VehicleDetail LEFT JOIN VehiclesToUpdate ON (VehicleDetail.SerialNumber = VehiclesToUpdate.SerialNumber) AND (VehicleDetail.Vin = VehiclesToUpdate.Vin)

Where VehicleDetail.SerialNumber is null

This returned no records which means that all of the cars are accounted for between VehiclesToUpdate and VehicleDetail. However, the number of records to update is still far short. I told you that I'm not giving up but I'm not sure what to try next.