Yes, there are quite a few out there. Some focus on the unique dynamic between Harry as a vampire and Jacob. They often explore how their relationship changes in this new, supernatural context. For example, in some fanfictions, they might start as enemies but gradually become allies or even more than that due to the strange circumstances.
Well, it's really hard to name the 'best' as it depends on personal taste. But some popular authors in the fanfiction community might be those who have a really good understanding of the Twilight universe. For example, some authors who have been around for a long time and have a large number of followers might be considered good. They often know how to write the characters of Bella and Jacob in a way that stays true to their original personalities while also adding new and exciting twists.
A popular one is 'Jacob's New Allies'. In this story, Jacob Black somehow finds himself in Mystic Falls and forms alliances with some of the vampires and witches there. It shows a different side of Jacob as he navigates this new and dangerous world of 'The Vampire Diaries'.
One common theme is the struggle between their different natures. Harry as a vampire and Jacob as a werewolf have inherent differences that can lead to conflict. Another theme is friendship or romance that might develop despite their differences. Also, the idea of acceptance, both of themselves in their new or different forms and of each other, is often explored.
I'm not sure if there is a specific novel with exactly this plot. It could be a very niche or even a fictional concept created for a particular discussion. There are many vampire - related novels in the world of literature, but this specific combination seems rather unique.
I highly recommend "Your Highness Vampire, Please Have Respect". The female protagonist of this fantasy and magical fantasy novel, Fanjo Man, was a vampire duke, but she was sent to a school for demons and vampires. The rulers of the demon world and the vampire world fell in love with her at the same time, but the heroine loved both of them. Later, the female protagonist mysteriously disappeared, and the demon ruler and the vampire ruler joined forces to find her. After finding her, they found that she was the person they were looking for. There were demons, vampires, and princes in this novel. It was exciting and romantic enough! I hope you like my recommendation. Muah ~
One interesting story could be where Jacob tries to convince Bella that she doesn't have to fully embrace the vampire life. He might show her all the beauty of the natural world she's about to lose. For example, he could take her to their old favorite spots in the forest. Another could be a story where Bella, as a vampire, has to protect Jacob from other vampires who see him as a threat because of his connection to her.
Another creative one is 'Jacob in the Vampire Diaries Universe: A Time - Travel Twist'. Here, Jacob Black is sent back in time to the early days of Mystic Falls. He has to deal with the original vampires and all the chaos that ensued back then. It's a really interesting concept that mixes time - travel with the two different universes.
In the fanfiction 'Jacob's Unwavering Imprint', the focus could be on how Jacob's imprint on Bella is unshakable despite the many difficulties. It might show how Jacob is willing to go against the norms of his pack and even face danger to protect Bella. This fanfiction could also explore how Bella starts to rely on Jacob in ways she never expected due to the imprint.
In 'Jacob imprints on a vampire' fanfiction, a common theme is the idea of transformation. Not just in the sense of physical transformation like the werewolf or vampire forms, but also in terms of personal growth. Jacob may have to transform his views on vampires because of his imprint. Also, the theme of sacrifice can be present. He might have to sacrifice certain aspects of his werewolf life for the sake of his imprinted vampire. And there's often the theme of a new world order. Their relationship could potentially change the relationship between werewolves and vampires as a whole.