Setting up a Vapor coding environment in VSCode on Windows
When running Vapor on Windows, WSL is essential. That means VSCode will also use files on WSL as the development environment.
Assuming VSCode is already set up, what you need is language-related support and a formatter. For language support, there is an extension simply named Swift, so I gratefully use it.

For the formatter, I gratefully used an extension called apple-swift-format.

After installing it, an alert appears if swift-format is not installed. So it needs to be installed, and I quickly installed it globally.
$ brew install swift-format
After that, the alert disappeared, and I could format code in a way similar to Prettier.
In a WSL and VSCode environment, Swift coding feels very smooth and comfortable. For some reason Xcode feels sluggish on my Mac mini, so I am starting to think I might use VSCode even on the Mac.