How to install VS code extension manually?

You can also use the command-line to install extensions from VSIX files using the --install-extension parameter.

code --install-extension /path/to/vsix

eg: code --install-extension vscodevim.vim

Download the extension from VSCode marketplace, it'll be a .vsix file, then do like the image below.

Just in case u have a same UI as mine. Happy coding!