Reddit reviews StarTech.com 10 ft Cross Wired DB9 to DB25 Serial Null Modem Cable - F/M - Null Modem Cable - DB-9 (F) to DB-25 (M) - 10 ft - SCNM925FM
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F/MNull modem cableDB-9 (F) to DB-25 (M)10 ftfor P/N: PEX1S553
The cable linked in the article should work fine.
That's not an RJ45 port, it's a MMJ connector. They're a special 6-pin connector used on most older DEC serial equipment. Pacific Cable still sells some MMJ cables.
By the time the VT510 was released, they were staring to phase them out, and use more IBM-PC compatible stuff. If you're connecting to old VAXes or early Alphas, then MMJ cables would probably be the way to go. Connecting with anything else, you'll want to use the DB25 port.
IIRC, you could hook up 2 different computers (1 on the DB25 port, and 1 on the MMJ) and then use some key combinations to switch between them.