Load JSON from a file in a Vagrant config file while defining a default structure. The file is created if it doesn't exist containing the specified default structure. No error checking is performed.

gistfile1.rb

# -*- mode: ruby -*-
# vi: set ft=ruby :

require 'json'
def load_json_default(filename, defaultstruct)
  if File.exist?(filename)
    json = File.read(filename)
    data = JSON.parse(json)
  else
    data = defaultstruct
    File.open(filename, "w") do |f|
      f.write(JSON.pretty_generate(data))
    end
  end
  return data
end

Vagrant.configure("2") do |config|
  config.vm.box = "precise64"
  config.vm.box_url = "http://files.vagrantup.com/precise64.box"
  
  config.vm.provider :virtualbox do |vb|
    vb.customize ["modifyvm", :id, "--memory", "256"]
  end

  config.vm.provision :chef_solo do |chef|
    chef.add_recipe "test"
    chef.json = load_json_default("settings.json", 
      {
        'test' => {
          'testuser' => 'my_user'
        }
      }
    )
  end
end