How to I make the content.json call for the original vanilla in game sprite
You just need to know where the vanilla sprite is located. All sprites should be located in
Characters/<character name>. For example, Abigail would be
Characters/Abigail. Keep in mind that other mods could overwrite that file during runtime so you won't have access to the original sprite unless your mod loads first.
Similarly, if you want portraits, that's in the aptly named
Portraits/<character name>. Example, Abigail would be
Portraits/Abigail.