Hmm.... in my case it seems to be related to the way components are imported in story files. The import path needs to include the/src/.

e.g.
import { Button } from "@acme/core/src";

The dynamic alias I'd set up in the storybook vite config was missing the /src 🤦‍♂️

I have story files alongside component files, slightly different file structure to the Turbo 'design-system' example. My components are packaged individually and also prefer importing my components via package name (like in the real world).

Anyway, got it working like this...

packages/components/button/src/Button.stories.tsx

import { Button } from "@acme/button";
...

apps/docs/.storybook/main.ts

import { vanillaExtractPlugin } from '@vanilla-extract/vite-plugin';

...
async viteFinal(config) {
  return mergeConfig(config, {
    resolve: {
      alias: [
        {
          find: /^@acme\/(.*)/,
          replacement: '../../$1/src',
        },
      ],
    },
    plugins: [vanillaExtractPlugin()],
  });
},
...

Feel free to let me know if there's another/better way 👍