One great 'loki laufeyson x reader fanfic' is 'A Dance with Mischief'. It has a really engaging plot where the reader character is drawn into Loki's world of trickery and magic. The author does a wonderful job of portraying Loki's complex personality and the developing relationship between him and the reader.
Well, Loki's magic is based on Asgardian sorcery, which is very different from the vampire's powers that are more biological in nature. However, they could still interact. The vampire's immortality might be of great interest to Loki, and he could study it using his magic. On the other hand, the vampire could use its power of rapid healing in conjunction with Loki's magic. For instance, if Loki is injured during a battle, the vampire could heal him faster by sharing its blood, and Loki could then use his magic to boost both of their defenses against their enemies.
First, understand Loki's character. Read the comics or watch the movies to get a good grasp of his personality traits like his mischievousness, his intelligence, and his underlying vulnerability. Then, create your reader character. Decide on their personality, background, and what makes them unique. Start with an interesting situation that brings Loki and the reader together, like a chance encounter in Asgard or on Midgard. Build the relationship gradually, with some conflict and resolution along the way.
Well, a plot might involve Loki and the original vampire getting into a power struggle. Loki, being a powerful Asgardian, and the vampire, with its own set of superhuman abilities. They could be vying for a magical artifact that has the power to either destroy or save their worlds. Or perhaps they start as enemies, but when a greater threat emerges, like an ancient evil, they are forced to team up. This could lead to a series of adventures where they learn about each other's strengths and weaknesses and gradually develop respect for one another.
In many such fanfics, the Avengers might find Loki through various means. Sometimes it could be intelligence they gather about the location where he is being tortured. For example, they might intercept a message from Loki's captors or notice some strange energy signatures that lead them to the place.
Thor would be a likely candidate. Since he is Loki's brother, he already has a connection with him. Thor might be the one to convince the other Avengers to give Loki a chance and be the first to reach out and try to befriend him in this new situation.
Future Loki might interact with the Avengers in a cautious way. He knows their past opinions of him and tries to prove himself. He could start by sharing small bits of useful information. For example, he might tell Iron Man about a weakness in a new enemy's technology.
There's a fanfic called 'The Loki - Avenger Alliance'. In this story, Loki is forced to team up with the Avengers due to a common threat from another dimension. It's full of action scenes and interesting dialogues between the characters, especially Loki's sarcastic remarks which are really in - character.