Vitest hoists every vi.mock call to the top of the file, before any import is evaluated. That is what makes the mock take effect for statically imported modules. It also means the factory function passed to vi.mock runs before any const in the file has been initialized.
Referencing a variable declared elsewhere in the file throws ReferenceError: Cannot access 'mockFn' before initialization, even though the code reads as if the declaration comes first. The same applies to imported helpers, since imports have not run either.
The supported escape hatch is vi.hoisted. Declare the shared values inside it, and the returned object is available to the factory: const { mockFn } = vi.hoisted(() => ({ mockFn: vi.fn() })). Alternatively, do the wiring inside beforeEach with vi.mocked(dep).mockReturnValue(...), which runs at normal time. Note that vi.doMock is deliberately not hoisted and applies only to imports evaluated after it, which makes it the right tool for per-test module behavior.