But the connector, which is like a DA-15 shell, with
just
three coaxial connectors inside ....
It's called a "3W3" connector. It, and variations some pins and some coax
connectors, were fairly common on workstations once upon a time.
A cable from 3W3 to 3xBNC should be easy to find. HOWEVER, none of the VAXstation-4000
graphics cards output anything like VGA video, and whether your PC monitor will sync up
with the VAXstation is a different question.
Bob