Custom VGA output : what resolutions are expected to work?
The question is not so much about what resolutions a monitor should support, the question should be about how to get a report of supported screen modes from the monitor to the PC. The monitor 'tells' the PC what modes are supported through I²C and the PC has to deal with that.

On Linux this looks like (notice I have 2 monitors attached): 

€ xrandr
Screen 0: minimum 320 x 200, current 3200 x 1080, maximum 8192 x 8192
VGA1 disconnected (normal left inverted right x axis y axis)
HDMI1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 510mm x 290mm
   1920x1080      59.9*+   60.0  
   1680x1050      59.9  
   1680x945       60.0  
   1400x1050      59.9  
   1600x900       60.0  
   1280x1024      75.0     60.0  
   1440x900       75.0     59.9  
   1280x960       60.0  
   1366x768       60.0  
   1360x768       60.0  
   1280x800       74.9     59.9  
   1152x864       75.0  
   1280x768       74.9     60.0  
   1024x768       75.1     70.1     60.0  
   1024x576       60.0  
   800x600        72.2     75.0     60.3     56.2  
   848x480        60.0  
   640x480        72.8     75.0     60.0  
   720x400        70.1  
DP1 disconnected (normal left inverted right x axis y axis)
HDMI2 connected 1280x1024+1920+0 (normal left inverted right x axis y axis) 376mm x 301mm
   1280x1024      60.0*+   75.0  
   1280x960       60.0  
   1280x800       74.9     59.9  
   1152x864       75.0  
   1280x768       74.9     60.0  
   1024x768       75.1     70.1     60.0  
   1024x576       60.0  
   832x624        74.6  
   800x600        72.2     75.0     60.3     56.2  
   848x480        60.0  
   640x480        72.8     75.0     66.7     60.0  
   720x400        70.1  
DP2 disconnected (normal left inverted right x axis y axis)


Unfortunately for this question I have only HDMI attached monitors, but classic VGA monitors talk back to the PC too when attached through 15 pin sub-D. Check DDC (Display Data Channel) on Wikipedia

Actually with --verbose flag you get all sorts of cool details but that is way beyond the scope of this question.

640×480 is a safe bet for all VGA monitors. So is 800×600 and most of the time 1024×768.