You could also use the extension emeraldwalk/vscode-runonsave https://github.com/emeraldwalk/vscode-runonsave
And then put the following in your settings.json:

  "emeraldwalk.runonsave": {
    "commands": [
      {
        "match": "\\.exs?$",
        "isAsync": false,
        "cmd": "cd ${workspaceRoot} && mix format ${file}"
      }
    ]
  },

This could also be useful for other commands which you want to run automatically on save.

I also recommend ElixirLS. But that alone will not format on save; in my experience.

To get that, just add this line to your settings.json:

"editor.formatOnSave": true

EDIT: This extension is recommended for vscode now: https://marketplace.visualstudio.com/items?itemName=elixir-lsp.elixir-ls

I’d recommend this extension instead: https://marketplace.visualstudio.com/items?itemName=JakeBecker.elixir-ls

It’ll do formatting for you among other things. Also, should be able to detect your .formatter.exs without any config.

Thank you very much for your comment! Although I am currently using ElixirLS, this looks like a fantastic extension and it seems very useful for lots of things