Sorry to hear the display did not respond to the manual override procedure. What are you using to communicate to the display?
Make sure you are communicating at 19200 baud with 8 bits data, no parity bit, one stop bit, and no flow control. If you are using a micro controller you may need to put the unit into TTL mode.
If you'd like to attempt a hardware repair, the X-Board
would be your backpack unit; the S-Series will provide a serial interface.
The fix won't be easy and will require some soldering skill. You'll have to remove the back communication PCB from the screen and clear all 14 mounting holes. You'll then solder the proper header to the X-Board and solder that to your screen. Finally, you'll program the board to work with a 20x4 VFD. All assuming the screen is still in good working order of course.
If you have any further questions regarding your VK204-25 or the X-Board, please don't hesitate to post.