The vent ID can be found on the serial number sticker in the battery compartment.
Remove the center magnetic faceplate
Across from the battery pack, is the serial number sticker. Just above the barcode you can find the Serial number (labeled S/N) and Vent ID (labeled ID).
Be careful not to press the raised reset button by accident. Doing so will reset the vent and disconnect it from your system. If you accidentally press the vent reset button, you can follow the steps in the article linked here.