How to auto format JSON on save in Vim

If you are keen on using external tool and you are doing some work with json, I would suggest using the jq:

https://stedolan.github.io/jq/

Then, you can execute :%!jq . inside vim which will replace the current buffer with the output of jq.

In one command, try this:

execute '%!python -m json.tool' | w  

You could then add you own key binding to make it a simpler keystroke. Of course, for this to work, you need to have Python installed on your machine.