You could also use the extension emeraldwalk/vscode-runonsave GitHub - emeraldwalk/vscode-runonsave: Visual Studio Code extension to run commands whenever a file is saved. · GitHub
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: ElixirLS Fork: DEPRECATED (use original extension) - Visual Studio Marketplace

I’d recommend this extension instead: ElixirLS: Elixir support and debugger - Visual Studio Marketplace

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