Before doing a factory reset (and you will lose anything that is not on an external SD card), try an internal reset. Turn off the Vox, carefully take off the back of the Vox and look for the round black button. Press the button using a paperclip or pen tip and the Vox will restart. See if your battery indicator is now fixed.
An underpowered wall charger will not power the Vox up, but I would think that it is no more likely to actually damage anything then attaching the Vox by USB to your computer. This is speculation on my part. Weird chargers seem to screw with the indicator
.