The video system on your PowerBook 520 is spaced to drive a VGA screen at 640x480 or 800x600 (8 bits only tho)
However you need two daisy-chained adapters to achieve that:
- PowerBook Display Adapter (part number 590-0831-A). That will gives you a Standard Mac DB15.
- Mac DB15 to VGA adapter (they usually have DIP switches). That will gives you VGA