Install Chef 11 with Vagrant's shell provisioner

Vagrantfile

Vagrant.configure("2") do |config|
  config.vm.box      = "ubuntu-12.04"
  config.vm.box_url  = "http://cloud-images.ubuntu.com/vagrant/precise/current/precise-server-cloudimg-amd64-vagrant-disk1.box"

  # Install Chef version 11
  config.vm.provision :shell, :inline => <<EOS
set -e
if ! command -V chef-solo >/dev/null 2>/dev/null; then
  curl -L https://www.opscode.com/chef/install.sh | bash -s -- -v 11.6.0
fi
EOS

  config.vm.provision :chef_solo do |chef|
    # ...
  end
end

@gildegoma Btw, I'm now using https://github.com/schisamo/vagrant-omnibus which also runs the install.sh script but is much easier to use. It even takes care of the curl/wget issue you ran into.