As previously noted I have experienced a battery discharging issue when the radio is left turned on in its fast charging station.
This problem seems to be reproducible, as I have now experienced it multiple times with two batteries (regular and high capacity). Of course, it's simple enough to remember not to leave the radio on like this, and instead to plug the power adapter directly into the radio if you want to leave it on. The radio's trickle charge circuitry seems to work perfectly fine in this regard.
Nevertheless, I feel that this issue with the fast charge shoe should either be fixed or documented - assuming that it isn't unique to my unit. Given that my unit works perfectly to charge batteries when the radio is off however it would seem that the basic charge circuitry is working - and perhaps this is indicative of a general design flaw/limitation, rather than my unit being broken in general. I suppose though it is possible that my unit is incorrectly configured to charge correctly when there's a concurrent load on the battery, but the fast charger is extremely compact and probably designed for simplicity without an fancy configuration of this sort. It would be interesting to learn from other VX-8R owners with a fast charger whether they have had a similar experience.