Field section. First thing to do is determine if the alternator is capable of outputting full output. It might have one or more bad diodes, bad stator, rotor, or worn brushes. "Rig" a clip / test lead(s).
Remove the GREEN field wire. Clip a lead from that alternator field terminal to ground. Turn the key to "run". Measure voltage at the OTHER field terminal, blue. It should be close to battery voltage
IF SO start the car, monitor battery voltage, and bring up RPM. Voltage should come right up, ammeter should show substantial charge, and V should climb above 13.5 and beyond. Turn on headlights, heater....With engine at fast idle, V should remain at least 13.5. or above. Don't rev engine and don't allow V to shoot past 15 by much.
If the above checks out OK, hook alternator wiring up "normal." Disconnect blue wire this time and ground that alternator terminal. Do NOT ground the blue wire, let it "hang."
Remove VR harness connector. Use screws (10-32? etc) to "make" terminals and jumper across the two connections in the VR harness connector. Repeat the above test. Should get same results.
===========================================
If the above tests show OK, scrape the VR clean and remount with star lock washers. The VR MUST be grounded. Inspect the VR connector and pins for looseness, corrosion. If this looks OK and it still won't charge, replace the VR