Well, Percy might be really confused at first. Mystic Falls has all those vampires and supernatural stuff, while he's used to Greek gods and monsters. He could probably use his water powers to his advantage, like putting out any fires started by vampires or something. Maybe he'd make some new friends among the Mystic Falls gang, or he could end up in a fight with some of the bad vampires who might see him as a threat to their power in the town.
Percy would probably have a hard time at first. He's used to the world of demigods and Greek mythology. But he's also adaptable. He'd learn about the vampires and werewolves in Mystic Falls. Maybe he'd start by making friends with some of the more normal humans in town to get a feel for the place.
Well, he might find himself in the middle of all the supernatural chaos in Mystic Falls. He could use his demigod powers to deal with vampires and werewolves. Maybe he'd form an alliance with some of the locals, like Elena or Stefan, and together they'd face new threats that neither has faced before.
Well, Percy Jackson's interaction with the Mystic Falls characters would be really interesting. He'd see the vampires' abilities as something different but not entirely unfamiliar, given his experience with all sorts of mythical creatures. He could start off by being a bit of an outsider, but as he gets to know them, he'd start to bond. For instance, he might help Bonnie with her magic, sharing his knowledge of the gods' magic. With Caroline, he might be amused by her perky and determined personality. And when it comes to Klaus, Percy would be on high alert, ready to use his powers if Klaus made any wrong moves, but also trying to understand his motives as he's used to dealing with complex and powerful enemies.
Percy going to Ilvermorny would be a wild ride. First, he'd be amazed by the castle itself. The four houses would be a new concept for him. He might be sorted into a house based on his heroic nature, like Thunderbird perhaps. He'd introduce the idea of Greek gods to the students there, which would cause quite a stir. And his relationship with Annabeth would be put to the test as they are separated by different magical institutions.
Another plot might involve Bella discovering new powers or abilities within the Mystic Falls setting. Since Mystic Falls has its own unique magical elements, it could enhance or change her in some way. She could also learn about different types of magic and how it differs from what she knew in her original world.
He might meet some talented witches and wizards. Maybe there would be a student who is an expert in magical beasts and they could share stories about their encounters with strange creatures.
Percy might meet an old enemy who has somehow been resurrected or changed in the future. This enemy could be more powerful and have a new plan to take over the world or destroy the demigods. Or he could meet a group of time - traveling demigods from an even further future who are trying to fix something in Percy's future that has gone wrong.
Percy might run into Loki. Loki is known for his trickery. He could either befriend Percy for his own devious plans or try to deceive him. Percy, being used to dealing with tricky situations in his own world, would have to be on his guard against Loki's schemes.