```ruby
Vagrant.configure(2) do |config|
  # install the Debian-provided Chef package
  config.vm.provision "shell", inline: <<-SHELL
     sudo apt-get -qq update
     sudo DEBIAN_FRONTEND=noninteractive apt-get -q -y install chef
     sudo gem install chef-vault
  SHELL
end
```

Use old enough chef-vault to support Ruby < 2.4
user
```ruby
Vagrant.configure(2) do |config|
  # install the Debian-provided Chef package
  config.vm.provision "shell", inline: <<-SHELL
     sudo apt-get -qq update
     sudo DEBIAN_FRONTEND=noninteractive apt-get -q -y install chef
     sudo gem install chef-vault --version '< 4'
  SHELL
end
```
assistant