Update, it turns out that the VSC warning was triggered by a failed O2 heater. Haven't found anyone that knows about a connection from the simple, only at engine start, test for low O2 heater current and the VSC. However, the VSC issue was noted on some of the discussions on the O2 sensor. Mine was the very common bank 1 (driver's side), sensor 1 fault, possibly common as the sensor is surrounded by some heat shields and probably runs hotter than the sensor on the other side. A bit painful to change the sensors, somewhat complicated by which orientation the connector is in. Mine connector was in an impossible position to push the tab, so I cut the connector on the sensor in pieces very carefully and pulled it apart. I would NOT recommend doing that, if you can't get the connectors apart the splice in a generic sensor. The O2 sensor fault was cleared immediately and a couple of engine starts plus a short drive later the VSC warning went away. Therefore, my old battery charger did not cause the VSC issue.