To use Drupal's multi-site features:

Add host to the config.json file with the document root as "drupal7" (or "drupal8") such as:

"themer": {
  "account_name": "root",
  "account_pass": "root",
  "account_mail": "box@example.com",
  "site_name": "Theme Development",
  "site_mail": "box@example.com",
  "vhost": {
    "document_root": "drupal7",
    "url": "themer.dev",
    "alias": ["www.themer.dev"]
  }
}

*Add a comma after the previous host or you'll get a config error.

Make sure you are pointing to the same document root as your drupal7 site is
Then re-provision the box and add your domain ("themer.dev" in this instance) to your host file.
Vagrant ssh again and type: drush @themer site-install (using your own domain of course).
If it does not work and you get an error about starting over, you might have to do a "vagrant provision" a second time.