Several of the Pokémon you encounter in Pokémon Brilliant Diamond and Shining Pearl will have evolution requirements. That means you need to give them an item, a move, or evolve them while they’re holding something during a specific time of the day. If you’re looking to evolve Vulpix into Ninetales, you need to make sure you meet particular criteria.
Before you go after Ninetales, make sure you’ve caught a Vulpix. It appears in the Grand Underground in caves with canyons, caves with lava, caves with sand, or the Tylpho Cavern when you’ve unlocked your National PokéDex after completing the Sinnoh PokéDex. It’s also important to note that Vulpix is a Pokémon Shining Pearl exclusive. You won’t be able to find it in the wild in Pokémon Brilliant Diamond, so you’ll need to trade with a player who owns Shining Pearl.
Now that you have Vulpix, the next step is to give it a Fire Stone. You can find a Fire Stone inside the Fuego Ironworks, or you’ll have to see it while exploring the Grand Underground. They spawn in the fossil digging minigame, so you’ll have to complete this minigame to find them. To increase your chances of these items appearing, make sure to collect the Diglett and Dugtrio bonuses that you’ll gain while walking through the passages.
Once you have the item, all it takes is to give Vulpix the Fire Stone, and it will immediately evolve into Ninetales.