The DEC VT420 terminal only has 6P6C MMJ connectors. We need a converter to make this work with a standard serial to USB adapter. The originals are hard to find or expensive, but we can assemble one new.
D9 Female to DEC MMJ. You need to wire up the DE-9/DB9 plug's wires to the adapter's MMJ jack yourself. They actually sent me a DE-9 male adapter by mistake, so I ended up just using an easily obtainable DE-9 gender changer once I'd wired up the adapter.
MMJ Null Modem Cable. You could also use a cable that isn't a null modem cable, and use a null modem adapter/cable on the DE-9 side. Or, you could wire up the D9 to MMJ adapter to be null modem.