How to include CSS from a vendor directory? Is it possible to include CSS from a vendor directory? I need to include a CSS artifact from a Pattern Lab dependency.

Is it best to copy the artifact into the Drupal theme and just reference it that way?
It's probably best to copy it, yes. Drupal isn't set up to serve any assets from the vendor/ folder, and actively prevents it by default with an .htaccess file. Also, all assets are normally handled via "libraries", which are defined by a module and whose JS/CSS files must be in the module folder.

You could change the .htaccess and insert a direct style element with /vendor/.../xyz.css into the page, but at that point it's probably a lot cleaner and more secure to copy it.

If it changes often and you want to avoid manually copying/committing it every time, you might look at automating this with a build script (eg. with webpack).