I haven’t tried this, but it should work in vDos, and perhaps also in vDosPlus:
In autoexec.txt include:
In config.txt change
font = WhateverFontNameYouUse
font = -WhateverFontNameYouUse
with a minus sign immediately to the left of “Whatever”
This causes vDos to display line-draw characters instead of whatever characters
are native to the font.
Again, not tested in vDosPlus.