According to their site, the Vox cannot be charged through USB connection. They also indicate that only the supplied charger can be used. I wish that charging cord was a lot longer!
You asked: "How do I charge my Kobo Vox".
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How do I charge my Kobo Vox?
Charging your Kobo Vox
While you can activate your Kobo Vox right out of the box, we recommend that you charge it for two hours before starting. It takes about five hours to charge your Kobo Vox completely.
To charge your Kobo Vox, connect it to the included wall charger and then plug the charger into a wall socket.
Note: You can only use the wall charger supplied by Kobo to charge your Kobo Vox. Do not use a third party charger. Check out the Kobo online Store to purchase accessories such as chargers.
Note: You cannot charge your Kobo Vox over USB.