There is firmware available online for some terminals; you could potentially get a lot more accuracy in emulating the actual firmware, but I'm sure a lot of that code gets into the guts of timing CRT cycles and other "real-world" difficulties. I'm not suggesting this would be easy to build out, just pointing out that it's available. While I haven't searched for the VT240 firmware, the firmware for the 8031AH CPU inside the VT420 (and a few other DEC terminals) is available on bitsavers. The VT240 has a T-11 processor, which is actually a PDP-11-on-a-chip.