This whole deal with markting certain VGA boxes as being able to play import/backup is pure BS. Any VGA box will work...all you need is the right boot disc.
Anyway, here is what you should do to test it. Turn off the DC. Insert the VGA box into the DC, then connect the monitor to the vga box. Switch the VGA box to "JAP" mode. Turn on the monitor. Turn on the Dreamcast without any disc in it. You should get the DC menu screen.
If you don't get the menu, check the pins on the VGA box AV connector. Make sure they extend all the way and evenly. If that doesn't show anything, confirm your monitor can do 31.5khz (with pc at 640x480).
If all else fails, you can open up the VGA box, (with all power to DC OFF) and use a multimeter to probe which pins are grounded. Check the gamesx site for which pins mean what....and make sure at least the 31.5khz wire is grounded. Its of if both the 31.5khz and 15khz wires are grounded. Still not right, check the JAP/USA/TV switch.