I think that Vdd being low when running on the linear is a feature of the linear circuit.
The ground of IC1 is connected to ground through the diode D102 and this lifts the output by the diode drop across D102.? If isn't taking much current so maybe 0.6V giving an output from IC101 of 3.9V.? This is reduced by the diode drop across D103, however D103 has a much higher current through it, maybe 100mA so will have a higher voltage drop, maybe 0.8V which gives a VDD of 3.1V.
Chris, G5CTH
