Disabling VT100 emulation in Mac Terminal
How can I turn off or disable VT100 emulation?


You can't. But you don't want to, either.

The DEC VT100 was an early video terminal for computer systems. It introduced a system of control codes which was widely copied by other terminals, and which has become a de facto standard. Virtually all computer terminal applications implement VT100 control sequences, and disabling them would make your terminal unusable.

That being said, this message is simply misleading. The resize utility is a weird little component of the xterm terminal emulator, which is distributed with XQuartz. XTerm is not the same as the Mac Terminal application, and it will always print this message when run with two arguments, regardless of what terminal you're using. You can skip this message and force it to resize your terminal by adding the -s flag, e.g.

/opt/X11/bin/resize -s 50 120


"s" stands for "Sun", as in Sun Microsystems, which had some terminal emulators which implemented extensions to the VT100 which included a sequence to resize the terminal window. (The VT100 didn't support any such sequence, as it was a hardware terminal and only supported two text resolutions: 80x24 and 132x24.)

Why this is necessary at all is… a good question, really. I suspect it comes down to both XTerm and the resize utility it ships with being very old pieces of software.