Zenith terminal & Vax Monitor

I have a Zenith Z-29 terminal hooked up to a 4000/VLC. No matter which 
term type I set the Zenith to, I can't get the monitor to accept most of 
the commands I put in. For example, "show dev" or "show ver" would both 
return "ILL CMD." To be honest, this is the first time I've used a real 
serial terminal. I'm pressing the "line feed" key to enter the command in. 
Pressing "return" simply seems to add spaces to the end of the current 
line, rather than entering it in the way that I would expect from a PC.

The "?" command does show a list of available commands. As stated before, 
all of them return "ILL CMD."

I can hook the Vax up to my PC to get through the monitor, but I'd like to 
be able to use this Zenith terminal for the full retro experience.