For a VS3100 M76 SPX the video cable is a BC23J. That's the cable I use
with my M76 SPX to connect it to an NEC MultiSync CRT. 15-pin to 3 BNC
connectors.
The SPX graphics controller is switchable between 60 Hz 1024 x 864 and 66
Hz 1280 x 1024.
http://home.iae.nl/users/pb0aia/vax/vs3khw.html