This happened to me a couple of weeks ago, too! My Vox seems to hold a charge fine if I leave it intermittently throughout the day (powered off or in sleep mode). I have left it turned off overnight or during the day several times with no I'll affect...but it's only been for less than 8 hours. Seems as soon as I leave it off for more than that, the charge gets eaten up.
Does anyone think it's possible for the battery to drain very slowly while turned off???