Simple Vagrantfile that checks for required plugins and installs them when they are missing.

Vagrantfile

# -*- mode: ruby -*-

required_plugins = %w(vagrant-exec vagrant-vbguest)

plugins_to_install = required_plugins.select do |plugin|
  !Vagrant.has_plugin? plugin
end

unless 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

Vagrant.configure(2) do |config|
  config.vm.box = 'ubuntu/trusty64'

  config.vm.provider 'virtualbox' do |v|
    v.memory = 1024
  end
end