According to [allpinouts.org][1] the video jack only provides composite output like the RF jack, so what you are looking for is a composite to VGA or DVI converter.  

Commodore C128/C64C Video Pinout:  
[![Commodore C128/C64C Video][2]][2]


  [1]: http://www.allpinouts.org/index.php/Commodore_C128/C64C_Video
  [2]: https://i.sstatic.net/pzISgUBf.png