This is the Vagrantfile I use for my day-to-day work for Juju development and testing.

gistfile1.rb

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

# Vagrantfile API/syntax version. Don't touch unless you know what you're doing!
VAGRANTFILE_API_VERSION = "2"

Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|

  # Every Vagrant virtual environment requires a box to build off of.
  config.vm.box = "trusty/juju"
  config.vm.box_url = "http://cloud-images.ubuntu.com/vagrant/trusty/current/trusty-server-cloudimg-amd64-juju-vagrant-disk1.box"
  config.vm.hostname = "jujubox"

  config.vm.provider :virtualbox  do |vb|

    # Gratiously borrowed memory and cpu code from:
    # https://stefanwrobel.com/how-to-make-vagrant-performance-not-suck
    # Give VM 1/4 system memory & access to all cpu cores on the host
    host = RbConfig::CONFIG['host_os']
    if host =~ /darwin/
        cpus = `sysctl -n hw.ncpu`.to_i
        # sysctl returns Bytes and we need to convert to MB
        mem = `sysctl -n hw.memsize`.to_i / 1024 / 1024 / 4
      elsif host =~ /linux/
        cpus = `nproc`.to_i
        # meminfo shows KB and we need to convert to MB
        mem = `grep 'MemTotal' /proc/meminfo | sed -e 's/MemTotal://' -e 's/ kB//'`.to_i / 1024 / 4
      else # sorry Windows folks, I can't help you
        cpus = 2
        mem = 1024
      end

      vb.customize ["modifyvm", :id, "--memory", mem]
      vb.customize ["modifyvm", :id, "--cpus", cpus]
      # increase the cpu cap
      # vb.customize ["modifyvm", :id, "--cpuexecutioncap", "50"]

      vb.customize ["modifyvm", :id, "--natdnshostresolver1", "on"]
    end


  ###########################################################
  # Configure routing so we can connect to lxc w/o sshuttle #
  # $ vagrant plugin install vagrant-triggers               #
  ###########################################################
  config.trigger.after [:provision, :up, :reload] do
      system('sudo route add -net 10.0.3.0/24 172.16.250.15 >/dev/null')
  end

  config.trigger.after [:halt, :destroy] do
    system('sudo route delete -net 10.0.3.0/24 172.16.250.15 >/dev/null')
  end

end