Vulpix in depth
Six tails and an onomatopoeia
The Japanese name, ロコン (Rokon), combines 六 (roku, “six”) with “kon”, the sound traditionally attributed to the fox in Japan. English gets to the same place by another route: vulpes, Latin for fox, plus six. Spanish didn't translate it, so the joke is still there without almost anyone noticing.
Two animals overlap underneath. One is the common red fox. The other is the kitsune of Japanese folklore, which gains tails over the years until it reaches nine, and which in older versions of the myth spits fire and lives for centuries. The Pokédex entries follow that rule to the letter: one tail at birth, six as an adult, nine after evolving into Ninetales. What the game changes is the tone. In the myth, the many-tailed fox deceives and devours; here, it becomes attached to its Trainer.
What arithmetic did to it
In Generation I, it had a single Special stat of 65. When that stat was split in two, Special Attack was left at 50 while Special Defense kept the original 65, meaning that a Pokémon that was already frail came out of the change even more harmless on the offensive side. It's one of those cases where splitting Special distributed things badly: the same number that had previously covered both damage and survivability became two figures, and only one of them stayed above average.
Drought, on the other hand, came from outside. It's a hidden ability that doesn't fit with anything on its stat sheet, and that's precisely why it worked: for an entire generation, the best sun support in the small-format metagame was a creature that couldn't win a single exchange by itself. Its job was to switch in, turn on the sun and die.
The other stone
In Generation VII, Sun and Moon gave it an Alolan regional form of the Ice type, with white fur and none of the original's flames. It was one of the first examples of the regional variant concept, and one of the cleanest: the same design, the same number of tails, the opposite reading of the same myth. That version evolves with an Ice Stone, not a Fire Stone, so Vulpix can evolve with either stone, depending on its regional form.
When to use the stone
Since evolution doesn't depend on level, the only reason to delay it is its level-up movepool, which stretches as far as 52 in Scarlet and Violet. Flamethrower comes at 32, Inferno at 48 and Fire Blast at 52; before that, the only move with decent power is Incinerate at 16, with 60.
Even so, in a normal playthrough, you don't get much out of battling with it. The combination that genuinely gets in the opponent's way is Confuse Ray at 20 followed by Will-O-Wisp at 24: confusion plus a burn on a physical attacker leaves the opponent hitting itself and losing HP every turn, and a level 25 Vulpix can do that to opponents well above its level. Nasty Plot and Energy Ball are available through TMs, which theoretically gives it an offensive plan with coverage against the Water- and Rock-types that stop it. In practice, boosting Special Attack twice from 50 still doesn't amount to much.