They look similar but a VGA port has 15 pins and a serial port has only 9. Serial ports like this one were really common before USB was used. You would plug peripherals into it kind of the same way you use a USB port. Mice were probably the most common use, but you could plug a lot of different things into them.
That depends on your goal. If you want something free, open source and self-hosted then Plex isn’t the best option and you’d do better to look at Jellyfin or Kodi.