The VSS on a 1991 Lumina V6 is typically located on the transmission case. It's a magnetic sensor that reads a reluctor ring on the output shaft of the transmission. You'll likely find it near the top or side of the transmission, but the exact location can vary slightly depending on the automatic or manual transmission type. You'll need to consult a repair manual specific to your vehicle's transmission (automatic or manual) for its precise location. A wiring diagram will also be helpful to confirm it.