How do you catch a vulpix in soulsilver?

You can't catch a Vulpix in Pokémon SoulSilver. Vulpix is a Fire-type Pokémon that is only found in the Pokémon Gold and Silver games, not in SoulSilver.

If you're looking for a similar Pokémon in SoulSilver, you might try catching a Growlithe, another Fire-type dog-like Pokémon that is available in the game.