By earliest, I mean earliest in the game, and by when I mean is there any good moves Vulpix learns that I should wait for?
The earliest you can find a Fire Stone is on Route 9. And I would suggest evolving Vulpix into the glorious Ninetales at Lv. 34, when it learns Flamethrower, or Lv. 39 if you want it to know Extrasensory.