In a VIN (Vehicle Identification Number) code, do the first 8 digits always refer to the same make/model, or does it change from year to year?
If everything stayed the same between two years (make, model, engine, options, origin, etc), you can expect that the only thing which would change between the two years is the 10th digit (year) and the last six which indicates the serial number of the vehicle.
The breakdown of any VIN of a car from 1981 until present sold in North America is as such (from Autocheck website):
This standard is covered under ISO 3833, but there are other competing standards throughout the world. Although, if a vehicle manufacturer wants to sell their vehicle in the United States, they will conform to ISO 3833.
If I find another VIN with the same WMI (5TD) and the same vehicle attributes (DKRFH), but the year and plant code are different, will that always still be the same make and model? (a Toyota Highlander in this example). Are there any exceptions?
For vehicles sold in North America, there should be no exceptions. In most cases the plant code will be the same amongst cars with the same vehicle attributes, because it becomes a lot more expensive for an auto manufacturer to have two plants rolling out the same cars. The logistics involved creates a greater expense for two plants. (It's a lot more economical to add more workers and/or assembly lines at the same plant than it is to run two identical plants.) Some manufacturers do assemble the same vehicles at different plants, such as VW, which does this with the Jetta (Germany and Mexico). The difference here would be, there would be a change in the first letter of the VIN as well as the plant being different.
Is WMI (world manufacturer id) + VDS (vehicle descriptor section) enough to identify the make & model, or is it necessary to include the model year also?
If you know a certain vehicle is a Toyota and you take the last eight to the dealership and ask them questions, they should be able to identify it with this information alone (they will cross reference the last eight with their database to see the whole picture of the vehicle in question). If you are trying to do it by internet search or on your own, you'll need to have the complete VIN in order to see what the vehicle make/model/year/etc is.