How to specify Vagrantfile?

Packer
1

I have created my Vagrantfile below

Vagrant.configure("2") do |config|
  config.vm.box = "bento/ubuntu-18.04"
  config.vm.network "forwarded_port", guest: 8080, host: 8080
  config.vm.network "forwarded_port", guest: 5901, host: 5901

  config.vm.provider "virtualbox" do |vb|
    # Display the VirtualBox GUI when booting the machine
    vb.gui = false

    # Customize the amount of memory on the VM:
    vb.memory = "2560"

  end
end

I want to use this Vagrantfile for created box.
My packer template JSON file is below

packer_template.json

{
  "builders": [
    {
      "communicator": "ssh",
      "source_path": "bento/ubuntu-18.04",
      "provider": "virtualbox",
      "type": "vagrant",
      "output_vagrantfile": "Vagrantfile"
    }
  ],
  "provisioners": [
    {
      "type": "shell",
      "script": "packer_script.sh"
    },
    {
      "type": "ansible-local",
      "playbook_file": "site.yml",
      "playbook_dir": "."
    }
  ]
}

Run packer build packer_template.json. This result is created output-vagrant directory.
This directory included Vagrantfile below

Vagrant.configure("2") do |config|
  config.vm.define "source", autostart: false do |source|
        source.vm.box = "bento/ubuntu-18.04"
        config.ssh.insert_key = false
  end
  config.vm.define "output" do |output|
        output.vm.box = "packer_vagrant"
        output.vm.box_url = "file://package.box"
        config.ssh.insert_key = false
  end
  config.vm.synced_folder ".", "/vagrant", disabled: true
end

So, executing vagrant up is launch output VM.
Then vagrant ssh result is This command requires a specific VM name to target in a multi-VM environment..

I’m using Packer v1.6.0.

Question:

  1. I want to vagrant up -> vagrant ssh without specify VM name.
  2. I want to use my Vagrantfile instead of the defined source and output VM.

How to?
Am I wrong?

Thanks.