One of Vulpix's Abilities is Drought, which automatically sets the sun. This turns Weather Ball into a BP 100, accuracy 100 STAB move, and while it's slightly weaker than Fire Blast, it's more accurate. If the Vulpix's Ability is Flash Fire, then Fire Blast (or Flamethrower/Overheat) should be used, but seeing as many have Drought, Weather Ball is more commonly used.
As for other weather, there are three instant weather setters: Drizzle, Sand Stream, and Snow Warning. No LC Pokemon have Drizzle. Out of the two with Snow Warning, one (Vulpix-Alola) is banned due to Hail + Aurora Veil, a deadly combo. The other (Snover), is murdered by Vulpix. Hippopotas has Sand Stream, but sand isn't very common in LC (none of the weathers are, but sun is the most used, from my experience).
Hope I helped!
