Walter,
I have a VM 100 chuck, and I think your VM 120 is similar.
Remove the two flat head screws from what VicMarc calls the "Lid" (and what I call the "Backing Index Plate"). It is the plate with the index holes and numbers. With the plate (LID) removed, you should find that there is a hole directly in line with the "pin". You do not need to remove the backing Jaw over-extension pin, you can just drive it into the chuck body, and when you are finished inspecting the chuck scroll ring, just drive the pin back out with a pin punch. You can also check for debris in the scroll ring / pinion drive area with the LID removed. Some years ago my VM 100 seemed to lock up while tightening or loosening the jaws, and during disassembly I found a tiny piece of steel broken from the end of the scroll ring that was getting hung-up in the backing jaw teeth during rotation. After removing the offending piece of metal, all has since been well with my VM 100 chuck.