The code looks good, but I'm not familiar with use case. For example, the purpose of force flag doesn't look obvious to me.

If this serves storybook's need, I'm good to proceed with this.

Looks good, great work! Once tests have been added this should be ready to go as is.

I tested this with two real-world use cases that developers building on top of Vitest have asked before:

Check if coverage is enabled via coverage.enabled or --coverage (or resolved by both)

import { defineConfig } from "vitest/config";
import type { Vite, VitestPluginContext } from "vitest/node";

export default defineConfig({
  plugins: [
    {
      name: "vitest:is-coverage-enabled",
      configureVitest(context: VitestPluginContext) {
        console.log({ coverage: context.vitest.config.coverage.enabled });
      },
    } satisfies Vite.Plugin,
  ],
});

Configure coverage.excludes from workspace project's plugins

import { defineConfig } from "vitest/config";
import type { VitestPluginContext } from "vitest/node";

export default defineConfig({
  test: {
    workspace: [
      {
        test: {
          name: "Headless unit tests",
          include: ["test/*.test.ts"],
        },
      },
      {
        test: {
          name: "Stories",
          include: ["src/**.stories.*ts*"],
        },
        plugins: [
          {
            name: "vitest:configure-coverage-for-storybook",
            configureVitest(context: VitestPluginContext) {
              // At this point we can assume `config.coverage` is resolved and defined to non-nullable
              if (context.project.config.coverage.enabled) {
                // No need to check if `coverage.exclude` is array, just push.
                // No need to care about default values like "**/node_modules/**", push doesnt overwrite anything
                context.project.config.coverage.exclude.push("**/*.stories.*");
              }
            },
          },
        ],
      },
    ],
  },
});