There could be a plotline where the vampire girlfriend is from a different dimension and she somehow ends up in the Naruto world. Naruto, being the kind - hearted person he is, takes her in and helps her adjust. As they spend time together, they fall in love. But then, her presence starts to cause some strange things to happen in the Naruto world, like strange disappearances or unexplained phenomena, and they have to figure out a way to stop it while also dealing with their relationship.
A common plotline might be that the vampire girlfriend has special powers that Naruto needs to defeat a powerful enemy. For example, her ability to control blood could be used in a creative way in battle. Also, there could be a plotline where their relationship is forbidden because of her being a vampire and Naruto being a ninja, and they have to fight against the norms of their respective worlds to be together. This would involve them facing opposition from both the ninja community and any vampire clans that exist in the fanfiction.