Here I post my findings about tolerance to frame height in lines, while line frequency is held constant, equal to one in standard VGA signal (640*480 @60fps -- 31.496kHz line frequency, or 31.75 microseconds per line).

 As the full frame at standard resolution contains 525 lines, I am changing it to lower and higher values, until the display cannot lock on it. With line frequency being constant, changing line number inversely affects frame rate -- the higher number of lines per frame, the longer it takes to draw it, and hence the number of frames drawn per second decreases.

At first, I am increasing number of lines - until the total number is no bigger than 530, the display accepts it as valid VGA 640*480, at 59fps. When I increase line count further, the recognized resolution falls back to 800*480. The most lines per frame before display couldn't lock to the signal is 620 lines, at this frequency. This translates to 50Hz frame rate.

Going to the other direction: the lowest line count when 640*480 is recognized is 522, lesser amount triggers fallback to 800*480. This goes down to 492 lines, and with less lines the lock is lost. Surprisingly, at 488 lines lock is regained, and now it recognized as 720*400px image, at 64 Hz. This lock is available at minimum line number of 421 per frame, at 75Hz. Any lower, and OSD is shown with message "Not Support".

----

From this and previous log I can conclude that the main constraint for this display is frame rate, which falls in range between 48 and 75 Hz. Anything above and below triggers "Not Support" message on screen, or it  just doesn't lock at all, and screen remains black. Seems like, when the frame rate falls in between these extremes, the line frequency and number of lines per frame can be changed freely. 

Also, when timings are close to VGA standard, the width to height ratio is chosen to be 4:3, as in the standard, and away from these values the monitor falls back to its native ratio close to 1.7 (1024 by 600 pixels of its LCD).