zulip/docs/development/setup/vscode-vagrant.md

If your preferred editor is Visual Studio Code, the Visual Studio Code Remote - SSH extension is recommended for editing files when developing with Vagrant. When you have it installed, you can run:

$ code .

to open VS Code connected to your Vagrant environment. See the Remote development over SSH tutorial for more information.

When using this plugin with Vagrant, you will want to run the command vagrant ssh-config from your zulip folder:

$ vagrant ssh-config
Host default
  HostName 127.0.0.1
  User vagrant
  Port 2222
  UserKnownHostsFile /dev/null
  StrictHostKeyChecking no
  PasswordAuthentication no
  IdentityFile /PATH/TO/zulip/.vagrant/machines/default/docker/private_key
  IdentitiesOnly yes
  LogLevel FATAL
  PubkeyAcceptedKeyTypes +ssh-rsa
  HostKeyAlgorithms +ssh-rsa

Then copy that config into your ~/.ssh/config file. You may want to change the host name from default to something more descriptive, like zulip. Finally, refresh the known remotes in Visual Studio Code's Remote Explorer.