Actually, the Vox only takes about 2 hours or so to go from nearly zero to full charge, so leaving it charging overnight is more than enough.
I would hope it doesn't "overcharge".
But I honestly don't know. I find the battery probably lasts as long as my laptop's battery if the Vox is in use, and that includes if I have it in sleep mode but with wifi on...and I have to leave the wifi on in sleep mode or it won't come out of sleep mode without a reset. So if I'm at home and I want to have quick access to the Vox, I'm leaving it in sleep mode but plugged into the charger because the wifi drains the battery all the time, even if the Vox is sleeping. I guess then there's no risk of overcharging if there is a constant drain on the battery anyway.