I don't have a schematic to share, but I've pulled down VTref (pin 1) by connecting it to GND (pin 4) via a 5.1K resistor. The jumper was in the right position, according to the photo I attached previously. J-Link Commander still measured 3.3V.
When the jumper is in the left position or removed, and the pull-down resistor is connected, J-Link Commander measures 1.12V, and the status LED switches to red instead of its usual green color.