What would happen if a male Eevee and a female Vulpix could mate?

No, they can't. Pokémon breeding has specific rules in the Pokémon universe. These rules are designed to keep the species somewhat distinct in terms of how they reproduce. Eevee is in the Field and Fairy Egg Groups, while Vulpix is in the Field Egg Group, but just being in the same Egg Group doesn't mean all Pokémon can breed. There are other factors and restrictions that prevent a male Eevee and a female Vulpix from mating.

The plot might involve the male Eevee being a bit of a wanderer and the female Vulpix being a protector of her den. Eevee stumbles upon Vulpix's territory. At first, Vulpix is defensive, but Eevee's friendly nature wins her over. The kiss is a culmination of their growing feelings. After that, they might team up to find a special artifact in the forest that is said to bring great fortune, all while dealing with other Pokémon who are also after the artifact.

After the kiss, they could start to explore their surroundings together more often. Maybe they find a secret place that only they know about and spend time there, sharing stories about their past experiences.

Maybe in the fanfiction, Eevee and Vulpix are both lost in a mysterious forest. They start to look for a way out together. Along the way, they come across ancient Pokémon ruins. Inside the ruins, there are puzzles and traps. Eevee, being a very adaptable Pokémon, uses its different forms' abilities (even if it doesn't actually change forms yet in the story) to solve some of the puzzles. Vulpix uses its fire to light the way and keep away any dangerous Pokémon that might lurk in the dark corners of the ruins. As they progress, they become closer friends and finally find their way out of the forest.

Friendship is a very common theme. Eevee and Vulpix are often depicted as having a strong bond, like in many fanfics where they support each other through difficult times. Another theme is adventure. They go on all sorts of quests together in different regions.

One common theme is friendship. Since Vulpix and Eevee are both cute and popular Pokemon, many fanfictions focus on their bond. They often go on adventures together, relying on each other.

Maybe Ash as a Vulpix gets adopted by a group of wild Vulpix. He has to learn their ways and how to survive in the wild. He could also learn some unique Vulpix - only skills that he can later use when he turns back to his human form.

The possible offspring of a male and female Eevee could be any of its evolved forms depending on various factors in the Pokémon world. For example, if it's exposed to a water stone, it could evolve into Vaporeon. Eevee has a diverse genetic makeup that allows for multiple evolutionary paths.

One possible fanfiction could be about Eevee and Vulpix teaming up on an adventure. They might start in a forest where they encounter a group of Pidgey that are causing trouble for the other small Pokémon. Eevee, with its adaptable nature, could use different moves to try and scare the Pidgey away, while Vulpix uses its fire - based attacks to create a barrier. As they work together, they start to understand each other's strengths and weaknesses better, forging a strong friendship along the way.

One popular theme is the journey they go on together. Maybe they're on a quest to find a special place or item, and their love grows during the adventure.