One common theme is the struggle between human and vampire nature. Since Dipper and Pacifica are the main characters, there might be a story where Pacifica as a vampire has to deal with her new instincts while still having feelings for Dipper, a human.
You can try searching on fanfiction websites like Archive of Our Own (AO3). It has a vast collection of different fanfictions, and you might find some great Dipcifica vampire ones by using the search function and typing in relevant tags.
Love is a very common theme. Most dipcifica fanfics explore the romantic relationship between Dipper and Pacifica. Another common theme is self - discovery. As they interact, both characters often learn more about themselves.
Adventure is also a big theme. Given the nature of Gravity Falls, fanfictions often send Dipper and Pacifica on various adventures. They might be exploring a new part of the forest, or uncovering a mystery that has been hidden for ages.
One common theme is the struggle with identity. Harry, being a vampire, has to deal with a new side of himself that he may not fully understand at first. For example, in many fics, he has to figure out how to balance his wizard self with his vampire instincts.
One common theme is the struggle between humanity and vampirism. When Ash is involved, it often shows his fight to keep his humanity intact even in the face of vampiric allure.
The theme of the outcast is also prevalent. Despite her power and allure, the vampire seductress is often an outcast in both the human and vampire worlds. Humans fear her, and vampires might be jealous of her power or see her as a threat. This sense of not belonging adds depth to her character and is often explored in different ways in fanfictions. For instance, she might try to find a place where she can be herself, or she might use her outcast status to her advantage by creating her own rules and living on the fringes of society.
Another theme is love and relationships. Since he's a vampire, his relationships with other characters, whether it's romantic or platonic, are often complicated. There might be a love interest who has to come to terms with Todoroki being a vampire, like in 'Todoroki's Vampire Love Affair' where the female lead has to navigate her feelings while also dealing with the danger that comes with Todoroki's vampirism.
A sense of otherworldliness is another theme. Vampire hypnosis fanfiction often plays on the idea that the vampire's world is different from the human world, and the hypnosis is a bridge between the two. In stories like 'The Otherworldly Hypnosis of Vampires', the hypnosis not only affects the human's mind but also gives them glimpses into the vampire's strange and alluring world, full of different rules, powers, and social hierarchies.
The theme of redemption can also be present. Sasuke may have a dark past as a vampire, and through his relationship with Sakura, he may seek redemption. This could involve him trying to control his vampiric urges or make amends for past actions.
In RWBY vampire fanfiction, a frequently seen theme is the clash of cultures. The RWBY world has its own set of values, and vampires bring in their own set of rules and behaviors. This can lead to interesting interactions and conflicts. Another theme is the search for a cure. If a character is turned into a vampire, they or their friends may be on a quest to find a way to reverse the transformation. And the theme of outcasts also exists. Vampires are often seen as outcasts in the RWBY world, and this can be explored in terms of how they deal with being shunned by society and how they form their own communities or try to integrate into the existing ones.