100 - ammeter not registering correctly is VR bad ?

Engine off, switch off, the needle of my Gauge is a little below the "halfway" dead center mark. Turn the switch on with the engine off, the needle drops further to the - side (to be expected).

Start the engine, and the needle still moves further on the - side, not above the halfway mark to the + side.

Is my gauge wired backwards ? Is my VR not adjusted properly ? How to check ?

Many gauges read a little off of Zero, so that is not a biggie if true.
Remove the negative battery cable and look at the gauge. If it stays in the same location, the needle is just a bit off. No problem there.
It should move farther to the left when the switch is turned on. Correct (as you indicate)
But the needle should move to the right when it is running (it will need to be at 3/4 throttle or full throttle to start the generator charging, then it should show charge. (usually about 8 to 10 amps for a while then tapering off to a bit above zero (or zero in your case)

Lets first polarize the generator.
With the battery connected, and the engine not running, use a stiff copper wire to bridge for one second, between the Gen terminal on the regulator (wire goes to the bigger generator terminal), and the Bat terminal on the regulator. This should make a nice spark (not dangerous).
Now the field poles are magnetized, and ready for charging.

Start it and see if it charges now. It might!

If not, make sure the gen and regulator have very shiny good grounds to the engine. Try it again!

If nothing, start it and set to 3/4 throttle. Use the above piece of wire to ground the F terminal of the generator o the engine. This will not hurt anything. If it now charges on the gauge, the regulator is probably bad. If it does not charge, take another piece of wire and (while it is running at 3/4 throttle, and the field is grounded as above, jump the Bat and Gen terminals (like you were polarizing, but with it running) If it now charges the regulator is bad. If it does not the generator should be tested at a reliable auto electric store. Many NAPA and other stores will do it free. Good luck, JimN