1. install vagrant-proxyconf plugin
set http_proxy=http://username:password@hostname:port
vagrant plugin install vagrant-proxyconf

2. configure the vagrant-proxyconf plugin
Create or edit file $HOME/.vagrant.d/Vagrantfile

Vagrant.configure("2") do |config|
  if Vagrant.has_plugin?("vagrant-proxyconf")
    config.proxy.http     = "http://username:password@hostname:port/"
    config.proxy.https    = "http://username:password@hostname:port/"
    config.proxy.no_proxy = "localhost,127.0.0.1,.example.com"
  end
end

3. download box and start vagrant
set HTTPS_PROXY=http://username:password@hostname:port
vagrant up