Charging system suddenly quit.

OK, next

Remove the regulator and jumper those two wires together. Just like hooking the field to 12V this should cause full output, but this time you are checking the wiring "end to end"

Also under the following conditions:

With wires jumpered together

and key in "run" but engine off

Check voltage at battery

and then check voltage at your jumpered connector and post them

Start it up, see if it charges as before

By the way the VR MUST be grounded. MUST

A new VR does not mean a "functional" VR

The only thing I can think of if the above test shows OK is.......

you got a defective replacement VR

or the alternator has / had a partial short in the field. You can measure alternator field current draw. Just hook the field to a battery with your multimeter in series, on DC amps

The fields vary over the years, but somewhere between 3 amps and "just under" 6 amps

Turn the pulley with your hand as you measure as the brushes are sort of intermittent