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.
Actually I have the VT240 firmware ROM dumps, that's where I got the original font from. The problem is, at least the VT240 is a rather sophisticated thing, with a T-11 CPU, some additional MCU, and a graphics accelerator chip. There is an extensive service manual available, with schematics and everything, but properly emulating the whole firmware + all relevant peripherals is non-trivial and a significant amount of work. The result is then a rather slow virtual terminal.
There is a basic and totally incomplete version of a VT240 in MAME though, which is good enough to test certain behavior, but it completely lacks the graphics part, so you can't use it to check graphics behavior like DRCS and so on.
EDIT: I also know for sure that there is a firmware emulation of the VT102 available somewhere.