I have now put VdosPlusMac on two Macbook Airs, one the pre-2017 with USB ports, the other the newer one with no USB ports (stupid Apple decision).
I copied over the XY folder, config.txt, autoexec.txt and my long-tweaked xy4-3.kbd file from the now entirely copasetic USB-equipped Macbook over to the newer model.
VDosPlusMac works nicely--thank you again Edward Mendelson and Kari Eveli--but there is a quirk on the 2018 Macbook Air.
To use XY4's classic assignments to the function keys on the newer Macbook Air--but not the older one--I have to add the Mac's fn. So instead of F1 or F5 in the older Macbook Air, it's fn-F1 or fn-F5 in the newer one.
Is there a line I can insert somewhere to always grab the Mac's fn key within the VDosPlus XY 4 window?
JP