>The VS3100 has a single BNC sort of connector, so
>I'm assuming it's monochrome. 

That's the ethernet connector - exactly where do
you have your monitor plugged in :-)

>In which case, can
>I use a DEC VT terminal in place of the monitor?
>I can get hold of a VT terminal quite easily, but
>I know no one with another monitor like my one.

You can change the VS3100 to boot without a
monitor (i.e. headless) by flicking switch S3
up at the back and connecting to the printer
port with your terminal. Typically 9600 8 bits
no parity is what you need.

Antonio