Simple Vagrant config to download and install the latest dev version of Drupal 8.
composer create-project thom8/drupal8-vagrant some-dir && cd $_
vagrant up
username: admin
password: admin
composer create-project -n thom8/drupal8-vagrant:dev-d7 some-dir && cd $_ && vagrant up
composer create-project -n thom8/drupal8-vagrant:dev-backdrop some-dir && cd $_ && vagrant up
This is an implementation of Beetbox so please refer to this project for more info.