IIRC, while the VT-220 had both 9-pin and 25-pin RS-232 ports, the 9-pin port was used to attach a printer. You also typically need a "null modem" adapter or cable to connect a terminal directly to a PC serial port (or USB serial adapter).
There's lots of original documentation for DEC VT series terminals here[1].
It has a 25-pin serial port which converts to a 9-pin serial port by a simple passive converter (basically, just ditch 16 of the pins and route the others properly, you can actually do serial with just two wires). Same goes for the ADM-3a
Them's fightin' words, boy.
DEC had a DE9 serial port connector whose pinout is not the same as the later IBM PC DE9 serial ports.