Starx

On the new version of Vagrant, answer by @Amos Shapira gets stuck in an infinite loop. The reason for that is each call to vagrant plugin install also processes the Vagrantfile and when processed executes the code relating to installing the plugin again and again and so on.

Here is my solution which avoids the loop.

# Plugins
#
# Check if the first argument to the vagrant
# command is plugin or not to avoid the loop
if ARGV[0] != 'plugin'

  # Define the plugins in an array format
  required_plugins = [
    'vagrant-vbguest', 'vagrant-hostmanager', 
    'vagrant-disksize'
  ]         
  plugins_to_install = required_plugins.select { |plugin| not Vagrant.has_plugin? plugin }
  if not plugins_to_install.empty?

    puts "Installing plugins: #{plugins_to_install.join(' ')}"
    if system "vagrant plugin install #{plugins_to_install.join(' ')}"
      exec "vagrant #{ARGV.join(' ')}"
    else
      abort "Installation of one or more plugins has failed. Aborting."
    end

  end
end