I've read that the flashing tool needs to be in windows XP, something that the driver is not compatible with Vista or windows 7. Or maybe you can run the flashing tool in "XP compatibility mode"? I'm not really a windows person so I'm not too clear on those details.
When you try and flash just with the Dingoo itself, you say nothing happens when you reboot while pressing the down button. Do you have the firmware file in the root of your internal memory? The file should be named a320.hxf and must be at the top directory on the internal memory.
When you turn the Dingoo on while holding the down button, watch the screen carefully. I have noticed that sometimes it will show a message then quickly go to the normal screen. For instance it can say something like "Same version" so it won't flash because it thinks you are trying to put the same version. I've also seen it say "firmware corrupted". Either way it does show the firmware update screen but only for a brief moment before it goes to the normal menu screen.
What do you mean when you say you "managed to flash dingoo on it" but you can't flash the firmware?