Kanto
Vulpix was given to Brock by Suzy in the episode Pokémon Fashion Flash. Suzy was Brock's idol, and Vulpix took a liking to him, which was surprising, as the little Pokémon had a very snobbish nature. It even liked Brock's homemade Pokémon food. It drove away Team Rocket with a very powerful Flamethrower, driving home the point that a focus on outer beauty could eclipse a Pokémon's inner strength. Impressed with the way Brock cared for his Geodude and Ash's Pikachu, Suzy decided that Brock might be able to take better care of Vulpix than she could.
Like many of Brock's Pokémon, Vulpix received very little screentime compared to the Pokémon of other main characters. When it was featured, Brock was often seen grooming and pampering it. This let to events in The Ninja Poké-Showdown, where Misty offered to trade her Psyduck for Vulpix, being sick of her Psyduck's headache and annoyed at Brock's compliments of Vulpix. Later the group met the Eevee brothers in The Battling Eevee Brothers who were throwing an evolution party for their younger brother Mikey to evolve his Eevee. They offered Brock a Fire Stone to evolve Vulpix but he refused, saying he didn't want to force Vulpix to evolve if it didn't want to.
Brock lent it to Misty when she was competing in the Princess Day Festival in Princess versus Princess and wanted a more balanced team (Ash also let her borrow Pikachu and Bulbasaur for the contest). It was seen defeating a Tangela with Flamethrower en route to the final. However, it was defeated easily by Lickitung's Lick.
Vulpix auditioned for the lead role in Pokémon in Love, Cleavon Schpielbunk's film, in Lights, Camera, Quacktion!. However, it abandoned the stage once it became apparently that the lead Wigglytuff was impossibly picky.
Brock also used Vulpix to help Ash train sometimes, as in The Ancient Puzzle of Pokémopolis, when it fought Pikachu in a training match, resulting in the two of them discovering the ruins of Pokémopolis. In this battle, it was revealed that Vulpix could use Quick Attack.
Vulpix was the only of Brock's Pokémon cloned by Mewtwo in Mewtwo Strikes Back and briefly fought its clone, before Mewtwo took all the clones away with him.
Orange Islands
When Brock decided to live at Professor Ivy's Lab in Poké Ball Peril, Vulpix resided there with him until his return in A Tent Situation.
Johto
Brock used Vulpix to fight Molly Hale's Teddiursa in Spell of the Unown, but it was defeated by the enormously powerful Pokémon's DynamicPunch.
Vulpix battled Miki's Skarmory in Hot Matches!. Its Flamethrower was dodged by Skarmory via Agility. Vulpix attempted to outrun Skarmory with its own speed, but Skarmory used Steel Wing on Vulpix, causing massive damage to Vulpix and knocking it out.
Brock later met up with Suzy again in Johto and returned Vulpix to her in Beauty and the Breeder, saying at the time that he had always felt that Vulpix wasn't really his and he was just looking after it for a time.