wlterm actually uses TSM as terminal-emulator. It emerged out of kmscon while I was looking for a terminal-emulator-library that is independent of any graphics-libraries and which does not render anything but only does vt-emulation. I didn't find one so I started TSM which is developed in-tree with kmscon and wlterm.
vt220 is almost fully supported and many extended feature are also available, but it doesn't pass all vttest tests. Things like double-width/height lines are still missing, but these are also rarely used today. Main focus was on making vim/emacs/mc/etc. work correctly (which currently do) and now we try to get full compliance to nearly all xterm features. But vttest is used heavily for testing.