Mame has VT240 support, I am not sure whether that is
compatible with
the VT125 or not.
Yes, I looked into MAME source code (it is available on Github and
Mamedev.org).
Yes, it has some VT125 codes but not enabled yet.
Also I found another VK100 emulation called "GIGI" - extended VT125 version but
it is flagged as not working at this time.
That came with BASIC interpreter itself and has programmable font images. They had
software at my university to teach students how to spell Russian words as foreign
language.
Tim