This is what KoboJason had mentioned previously:
Originally Posted by KoboJason
The Kobo Vox is designed to charge the most efficiently with the included wall charger.
Although we don't recommend using a 3rd party charger, the Vox will charge using a 3rd party charger provided:
‣ the charger used is a wall-charger and not a standard USB cable
‣ the charger supplies 2 amps
Note the charging light will not show the reader is charging when using a 3rd party charger but know it's still receiving a charge.