---
- hosts: blog
  user: vagrant

  tasks:
  - name: Copy private key
    copy: src=~/.ssh/id_rsa dest=/home/vagrant/.ssh/id_rsa mode=0600

---
- hosts: blog

  tasks:
  - name: Copy private key
    copy: src=~/.ssh/id_rsa dest=/home/{{ ansible_ssh_user }}/.ssh/id_rsa mode=0600

Use ansible_ssh_user instead of vagrant user
Use ansible_ssh_user instead of vagrant user
