The problem is that pool: 'threads' runs each worker in its own isolated Node.js thread with separate module caches. Your setupFiles executes once per thread, so globalThis._vuetify gets recreated in each one — there's no shared Vuetify instance across threads.

Three approaches:

1. Switch to pool: 'forks' with isolate: false (recommended)

test: {
  pool: 'forks',
  isolate: false,
}

forks is actually the Vitest default. With isolate: false, module caches are shared across tests within a single worker process, so your singleton-in-setupFile pattern works. The setup runs once per worker (not once per test file), and Vuetify gets reused.

2. Use globalSetup + provide/inject

globalSetup runs once in the main process before any workers. You can create Vuetify there and pass serializable config to workers:

// vitest.globalSetup.ts
import type { TestProject } from 'vitest/node'
export default function setup(project: TestProject) {
  project.provide('vuetifyOptions', vuetifyOptions)
}

declare module 'vitest' {
  export interface ProvidedContext { vuetifyOptions: any }
}

Then in setupFiles, inject the options and recreate Vuetify. This helps if the creation is expensive, though workers still each have their own instance.

3. Accept per-worker recreation

If Vuetify setup isn't actually expensive, recreating it per worker may not matter. Your globalThis caching already prevents recreation within a single thread, which is the most common case.