authorized_keys.yml

---
- hosts: foo
  tasks:
    - command: ssh-keygen -f ~/.ssh/id_rsa -P ''
    - command: cat .ssh/id_rsa.pub
      register: key

- hosts: bar
  tasks:
    - authorized_key: user=vagrant key="{{ hostvars[item].key.stdout }}"
      with_items: groups['foo']

Vagrantfile

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

VAGRANTFILE_API_VERSION = "2"

Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
  [
    {name: 'foo1', ip: '192.168.77.21'},
    {name: 'foo2', ip: '192.168.77.22'},
    {name: 'foo3', ip: '192.168.77.23'},
    {name: 'bar1', ip: '192.168.77.24'},
    {name: 'bar2', ip: '192.168.77.25'},
    {name: 'bar3', ip: '192.168.77.26'},
  ].each do |m|
    config.vm.define m[:name] do |c|
      c.vm.box = "sl65"
      c.vm.network "private_network", ip: m[:ip]
    end
  end

  config.vm.define "foo1" do |c|
    c.vm.provision "ansible" do |ansible|
      ansible.playbook = "site.yml"
      ansible.limit = 'all'
      ansible.groups = {
        "foo" => %w(foo1 foo2 foo3),
        "bar" => %w(bar1 bar2 bar3)
      }
    end
  end
end