If you think it’s timing then you need to measure it and see what’s going on.
The vernier adjuster is not part of the vacuum advance; it simply allows you to make small corrective movements of the distributor without undoing the base plate.
If it is stuck then maybe other things inside your dizzy are too. I suggest you take it out, strip it and clean it and make sure that the advance plate turns and the weights move freely.
Now with the dizzy held in your vice measure the total advance available by turning the shaft with the rotor arm on it. I made up a 360 degree protractor to do this but you should be able to measure it well enough. It should be around 12 degrees depending on car and distributor. Correct numbers are in the Manual. Total advance = distributor advance x 2 + static advance. = 12x2+10 = about 34.
To measure the advance curve at speed you will need to buy a dial back timing light. Google them. Cheap ones are about $50 and you’ll wonder how you ever lived without one.
Simply get an assistant to rev the engine at say idle, 1000, 1500, 2000, 2500, 3000 and 4000 and measure the crank advance using the dial back light. You can then plot your curve, and compare it to original. Then you will know if your advance is correct.
Here’s one I did for my 3.8.