Solution:

In order to allow single-line blocks in VSCode using Prettier - Code formatter extension, please take the following steps:

1. Enable stylelint integration by adding this in the VSCode Settings (JSON): "prettier.stylelintIntegration": true
2. Install stylelint and stylelint-prettier npm modules in your project directory. npm install stylelint stylelint-prettier --save-dev
3. Add a .stylelintrc.json file at the root of your project directory with the following code:

    {
        "plugins": ["stylelint-prettier"],
        "rules": {
            "block-closing-brace-newline-after": "always-multi-line",
            "block-closing-brace-empty-line-before": "never",
            "block-closing-brace-space-before": "always",
            "block-opening-brace-space-after": "always",
            "block-opening-brace-space-before": "always",
            "block-closing-brace-newline-before": "always-multi-line",
            "block-opening-brace-newline-after": "always-multi-line",
            "indentation": 4
        }
    }

You can add/customize more stylelint rules, see the entire list of rules here.

Took me a while to understand how to configure these options, if you're starting out with stylelint, I highly recommend you read its guidelines first.

Since you are using prettier to format your document, its easier to configure it according to your needs.

This behaviour is set by the setting "prettier.printWidth: 80" it is 80 by default and you can increase it as you like so that it wont break up the line.

You can set the value in your settings.json by adding "prettier.printWidth": 120, or in the UI (Prettier Extension Settings)

You can increase it to whatever you like but beware this will apply to all languages you let prettier format with.

to disable prettier for css you can add it in th settings here:

or this in settings.json

"prettier.disableLanguages": [
    "css"
]

As mentioned in comments, OP doesn't want validation of his CSS to occur

to do that set "css.validate": false in settings.json

Don't know why but setting Default Formatter to ebsenp.prettier didn't work for me. But I found a similar command that worked.

1. ctrl + shift + p
2. Format document with
3. Configure default formatter
4. Choose prettier

Yes, try installing vscode-css-formatter extension.
It just adds the functionality to format .css files and the shortcut stays the same Alt+Shift+F.