There shouldn't be any null records in the serial number column. But you made a good point. The join was wrong. It should have been:

Where VehiclesToUpdate.SerialNumber is null. When I did that, 157,511 records came up. But there seemed to be no rhyme or reason for these. I'll do some more invesitgation.

As for the stats. 209,809 total records in VD. Only 177,303 records came up in the VehiclesToUpdate. That leaves 32,506 not 157,511. We may be dealing with duplicates, which I will also investigate and let you know.