Hi ArcadeBliss,
You don't have to enter any modeline, but a crt_range line (completely different thing). Modify the correct line in the d9400 preset (monitor.ini in VMMaker's folder):
crt_range2 20501-29000, 40-80, 2.910, 3.000, 4.440, 0.451, 0.164, 1.048, 0, 0, 320, 384, 0, 0
by
crt_range2 20501-29000, 40-80, 2.910, 3.000, 4.440, 0.451, 0.164, 1.048, 0, 0, 320, 384, 640, 768
By doing this, you unlock interlaced modes for the 25 kHz (EGA) range. Then you'll need to add the 1024x768 mode to user_modes.ini so VMMaker calculates it.