Edit mame.ini with these settings:
monitor_specs0 15250-16500, 50-80, 2.187, 4.688, 6.719, 0.190, 0.191, 1.018, 0, 0, 288, 448
monitor_specs1 23900-24420, 50-80, 2.910, 3.000, 4.440, 0.451, 0.164, 1.048, 0, 0, 384, 768
monitor_specs2 31000-32000, 50-80, 0.636, 3.813, 1.906, 0.318, 0.064, 1.048, 0, 0, 576, 768
monitor_specs3 37000-38000, 50-80, 1.000, 3.200, 2.200, 0.020, 0.106, 0.607, 0, 0, 600, 768
This will force every refresh to be above 50 Hz.
If obtained refresh is too different from native refresh the vsync option won't be activated, that's why you still see 100%.