I have a VXT2000 model VX20A-AD, I guess the puniest of 'em....6mb RAM.
Some on sale on ebay these days...
It has a pluggable framebuffer, model DWT II MONOCHROME. It gives me
gibberish on b/w (vert sync) and on a DEC  VRM17-HA screen (Paperwhite
grayscale sync-on-green). I understand it works with a VRE315 monitor.

Q: Are there other framebuffers? Does anyone know the modes of this
particular framebuffer?

Grumble grumble: Who will swap a VT1200 for a  VXT2000? I'm still trying to
get that VRE01 monitor to work at 1024x764 1bit in dithered mode...

Fred