The VIN on a 67 was inside the door, which was closed for 90 percent of the trip thru the GM side of the plant so it wouldn't be visible, so I doubt the number has any meaning to the factory itself. GM used the W/O number that is noted on the body and chassis broadcast sheets that was hand written onto the firewall of the tub, and the various parts of the front end clip with grease pencil to track the body thru the plant. All of the other parts specific to that car, ie not just a base part that had to go on every car, had tags attached to them with that number on it as well.
This made sure that each part for a particular car got onto that car. The W/O number was assigned as the car was released from the body bank, and it was a sequential number starting at 0 and progressing forward throughout the day, then resetting to 0 at the start of the next day. It would be alot easier to see a 2 or 3" high w/o number on the firewall than to have to run over and look at a tag that was inside the door in the case of 67's, or on the dash for 68's and 9's as the car was coming into your station. Plus those numbers should be sequential since they were assigned as the cars left the body bank, while the VINs may jump around abit since they were assigned sequentially as the car came into the bank. Depending on the options on a particular car it may sit for a while in the bank before being released so the VINs get jumbled around some as they leave.