In fanfiction, Naruto characters would have to adapt to the different power systems. In the Rosario + Vampire world, there is magic and different vampire abilities. A character like Rock Lee would have to learn to use his taijutsu in a world where there are flying creatures and magic spells. Meanwhile, the Rosario + Vampire characters might be intrigued by the Naruto characters' use of chakra. They could try to learn it, which would create a whole new set of stories and interactions.
Well, think about it this way. In the Rosario + Vampire universe, there are many different species and hierarchies. Naruto characters could disrupt or integrate into these. For example, Kakashi could use his intelligence and combat experience to navigate the complex politics of the Rosario + Vampire world. He might end up being a mediator between different groups. And the Naruto characters' values of loyalty and friendship could have a big impact on the relationships in the Rosario + Vampire world.
Well, he could fit in as a unique addition to the cast. His ninja abilities would be quite different from the normal powers in the Rosario Vampire world. He might be seen as a wildcard, with his speed and combat skills being a big advantage.
Naruto could be a new student at the Yokai Academy. His presence with the Kyuubi's power would disrupt the balance. For example, the vampires might see him as a threat or a potential ally. He could use his shadow clones to spy on different factions, like the witches or werewolves, to understand the politics of the school better.
I'm not sure if there is such a specific fanfiction where a character from Rosario Vampire is reading Naruto fanfiction. Fanfiction can be very diverse, but this seems like a rather unique and specific concept. It's possible that it might exist in the vast realm of fan - created stories, but it would be quite difficult to find without a more targeted search.
Shirou's projection magic could be really interesting in the Rosario Vampire world. He could project weapons that are effective against the various supernatural beings there. For example, if there are werewolves, he could project a silver - edged sword.
One idea could be a crossover where Naruto characters end up in the Rosario + Vampire world. For example, Naruto could be mistaken for a powerful yokai due to his chakra. Sakura might study the unique magic in the Rosario + Vampire world. Another could be a vampire in the Naruto world, using their powers in a different way than the normal jutsu.
The characters would have very interesting interactions. Naruto would probably be curious about the vampires and their powers. The vampires, on the other hand, might be wary of Naruto at first. And if there's a dragon character, it could be a source of awe for everyone. For instance, Naruto might try to befriend some of the Rosario Vampire characters, while they might be hesitant because of his different nature.
Maybe Naruto forms an alliance with some of the main characters in Rosario Vampire. He could use his ninja skills combined with his vampire abilities to fight against common enemies. Like, if there's a powerful dark force threatening both the vampire and human worlds, Naruto could be a key player in the defense.
Tsukune could fit in as an outsider with unique abilities. His vampire powers would be a contrast to the ice - based powers in the Sub Zero world. He might be initially seen as a threat or a curiosity.
Naruto could be seen as a unique kind of 'monster' in the Monster High world. His ability to use chakra and perform ninjutsu would be very different from what the Monster High students are used to. He might befriend some of the more outcast characters, like Frankie Stein, because he also knows what it's like to be different. His positive attitude would also bring a new energy to the Monster High school.
Sasuke could be seen as a kind of dark and brooding figure that the Addams Family might respect. His quest for power and revenge could be something they find interesting. Naruto, on the other hand, with his bright and positive attitude, would be a contrast to the Addams Family's usual aesthetic but could bring a new kind of energy to their world.