Its because the Steering Angle Sensor needs to be reset. VDC uses many inputs to work including steering angle. Disconnect your battery and the sensor looses its 0deg point which is straight. It will relearn itself but in order to do so the vehicle has to be driven straight and the wheel can not be turned for several minutes. So find a long straight road. We at the dealer can reset this sensor with our scanner in minutes. Next time leave battery connected, its just head lights.