Fleur could use her Veela charm to try and disarm or distract vampires. Since Veela have a certain allure, it might work on vampires as well.
Fleur could also interact with vampires on a more empathetic level. If she met a vampire who was once a wizard or had some connection to the wizarding world, she might try to understand their plight. For example, a vampire who was turned against their will and is now shunned by both the wizarding and Muggle worlds. Fleur could try to find a way to help them co - exist or at least not be a threat to the wizarding world.
Fleur and Tonks might start off a bit wary of each other. Fleur, with her proud and somewhat haughty nature, could see Tonks as a bit too brash. But as they both love Harry, they would have to find common ground. They could start sharing stories about Harry's heroic deeds from their different perspectives. Eventually, they would become friends and support each other in the relationship with Harry.
In many fanfics, Fleur is often depicted as strong - willed. She's a Veela, so she has that alluring quality, but she's also smart and not just a pretty face. For example, in some fanfics she uses her Veela charm in a strategic way to get what she wants or to protect others.
In this fanfiction, the vampire could be an old acquaintance of Fleur from her Beauxbatons days. He was once a student there who got turned into a vampire later. Harry gets involved when he notices some strange happenings around Fleur. The three of them then get caught up in a mystery that involves a powerful dark magic item. They must work together, with Harry using his Gryffindor courage, Fleur her Veela charm, and the vampire his immortality - related skills to solve the mystery and keep the item from falling into the wrong hands.
At first, they might be at odds with each other. Fleur could think Harry is too impulsive, while Harry might find Fleur a bit too haughty. But then, when they are forced to work together on a mission, they see the good in each other. Fleur realizes Harry's selfless nature, and Harry sees Fleur's intelligence and determination. This new - found respect leads to a romantic relationship and ultimately marriage.
For Harry, you could focus on his sense of justice. Maybe he's initially wary of the vampire but his experiences with Voldemort have taught him not to judge too quickly. Fleur can be developed by highlighting her Veela allure and how it affects the vampire. The vampire could be developed by giving him a tragic backstory. For example, he was turned against his will and is trying to find a way to reverse it. This could create an interesting dynamic between the three characters.
In some fanfictions, Harry Potter might help Fleur by using his knowledge of magic. For example, if Fleur is in a dangerous situation during a wizarding tournament task, Harry could share some of his unique insights on how to overcome the magical obstacles, having experienced similar challenges himself.
Well, perhaps Fleur rejects Harry for some reason. Maybe she's still in love with Bill and tells Harry she can't be with him. This would leave Harry heartbroken as he might have developed deep feelings for her.
A possible plot could be that Harry Potter discovers a prophecy that involves Fleur and a powerful vampire. The prophecy states that only together can they defeat a new dark force rising. Along the way, Fleur has to struggle with her own identity as she is part Veela and now has vampire traits after an encounter, and Harry has to deal with the mistrust of the wizarding community towards Fleur's new state.
He might become withdrawn. Harry has been through a lot in his life, and being heartbroken by Fleur could make him retreat into himself. He may avoid his friends and isolate in his dormitory at Hogwarts.
One possible plot could be that Harry and Fleur run away to escape the chaos of the wizarding world. Maybe they go to a far - off land where they can live in peace and explore magic on their own terms. They might discover new spells and magical creatures together.