One common theme is the forbidden love. Since vampires are often seen as different and dangerous, their love with a human or even another vampire from a different clan is usually considered forbidden. Another theme is the struggle between good and evil. Vampires are sometimes portrayed as both having a dark side but also being capable of love, which creates an internal struggle. Also, immortality is a theme. The vampire's immortality affects their love relationships as they have to deal with seeing their loved ones age or die while they remain the same.
A really interesting cartoon vampire love story is 'Dance in the Vampire Bund'. It's set in a world where vampires have revealed their existence to the world. Mina Tepes, the vampire queen, is in love with Akira Kaburagi Regendorf. The story delves into their relationship while also exploring the political and social aspects of a world where humans and vampires co - exist.
To write a compelling vampire story in this setting, pay attention to the atmosphere. Make it creepy and enchanting at the same time. Develop complex relationships between the characters. And don't forget to include some thrilling action scenes.
Loneliness can make a vampire sad. Since they often have to live apart from humans due to their nature, they lack real companionship. For example, a vampire might see humans enjoying family and friends but can't fully be a part of it.