How do I add my own public key to Vagrant VM?

You can use Ruby’s core File module, like so:

  config.vm.provision "shell" do |s|
    ssh_pub_key = File.readlines("#{Dir.home}/.ssh/id_rsa.pub").first.strip
    s.inline = <<-SHELL
      echo #{ssh_pub_key} >> /home/vagrant/.ssh/authorized_keys
      echo #{ssh_pub_key} >> /root/.ssh/authorized_keys
    SHELL
  end

This working example appends ~/.ssh/id_rsa.pub to the ~/.ssh/authorized_keys of both the vagrant and root user, which will allow you to use your existing SSH key.