Problems

Vagrant leverages a insecure public/private keypair to create base boxes so that vagrant installations can automatically SSH into the boxes.

By default, vagrant up generates a new public/private keypair locally and inserts this public keypair into the virtual machine.

The private key is stored in the file - .vagrant/machines/default/virtualbox/private_key.

Naive Solution

You can insert a newline config.ssh.insert_key = false after config.vm.box, e.g.,

Vagrant.configure("2") do |config|
  config.vm.box = "debian/stretch64"
  config.ssh.insert_key = false
end

[1] Insecure Keypair