One common theme is identity. Harry has to figure out who he is as a vampire while still being Harry Potter. Another theme is loyalty. His vow often tests his loyalty to his friends and the wizarding world.
Well, there's often the theme of secrecy. In the wizarding world of Harry Potter, secrets are a big deal. When you add vampirism to the mix, Harry and Severus would need to keep that a secret even more. Also, the theme of redemption can be present. Maybe Severus is trying to redeem himself through his vow to Harry who has become a vampire. Or Harry is seeking redemption for some past actions in his new vampire - like state. And of course, the theme of forbidden love can also crop up in these fanfics, as the relationship between Harry and Severus is already complex and adding vampirism can make it even more so.
One great 'vampire harry vow fanfic' is 'The Vampire's Pledge'. It has an interesting plot where Harry discovers his vampire heritage and has to make a vow that changes his life. The story is full of action and mystery.
There are several places to look for such fanfics. Archive of Our Own (AO3) is a great resource. You can simply type in 'vampire harry severus vow' in the search bar and browse through the results. Some fanfics might explore the relationship between Harry and Severus in a world where vampires exist and there are special vows involved. Another option is Fanfiction.net, but AO3 generally has a wider range of fan-created content.
One creative interpretation could be that the Unbreakable Vow is used in a fanfic to bind two characters in a secret alliance against a new dark force. For example, two students from different houses might make the vow to protect a powerful magical artifact from being stolen by a group of dark wizards who are trying to use it to gain control over Hogwarts.
There's a good chance. Fanfics are known for their creativity and bending of established norms. If the fanfic is centered around this idea, then Harry could very well be an ancient vampire. This could lead to interesting plotlines involving his vampiric powers, his interactions with other characters who may or may not know his true nature, and his struggle with his new identity.
In the original series, the Unbreakable Vow is a very serious and rarely used magical bond. In fanfic, it can be more common. For instance, in fanfic authors might use it more often to drive the plot forward. They might have characters enter into the vow more casually (compared to the original where it was a big deal).
Well, in such a fanfic, Harry might have enhanced physical abilities like super strength and speed which are typical of vampires. He could also have a more commanding presence, being 'dominant' perhaps in his interactions with other characters, whether it's in a social or a more power - related context.
Well, he might have super strength and speed, which are common traits among vampires. He could also have enhanced senses like acute hearing and vision. And perhaps he has the ability to charm or hypnotize others.
Love and relationships are also common themes. His vampiric nature can affect his relationships with friends like Ron and Hermione. Some fics explore romantic relationships where his vampire side adds an element of danger or mystery. And then there's the theme of isolation. Harry may find himself isolated from the wizarding community due to his new nature, which leads to his struggle to find his place again.