The Vox charger has a custom pinout that allows the device to recognize when it is connected to it's own charger and report charging status. So, 3rd party chargers won't work properly in that respect.
That said, a third party charger, as long as it is 5v, 2a WILL charge the battery. You just won't see the charging indicator stuff. But if you look at the battery status area of the settings, you can confirm that it is taking on charge.