Three miscellaneous improvements for Vim

All these settings except `softtabstop` are already applied now due to #15429. And I don't think it makes sense to use a variable to disable those; if someone wants to disable them they can just use an `after/ftplugin/rust.vim` file to set it back to whatever they want for Rust.

As per @blake2-ppc's suggestions on #15429, adding in the `softtabstop` setting makes sense, but it should be done with the other settings.