One common theme is the forbidden love aspect. Since Sasuke and Hinata are from different clans in the Naruto universe, adding the vampire element makes it even more complex and forbidden. Another theme could be the power struggle. If Sasuke is a vampire, he might have powers that are difficult to control, and Hinata could be the one to help him. Or vice versa.
Well, a frequent theme is the transformation. If Sasuke or Hinata (or both) are vampires, their transformation into vampires and how they adapt to their new nature is often explored. Also, the theme of loyalty comes up. How loyal are they to each other in this new and dangerous situation? And there's also the theme of secrecy. Vampires usually have to keep their identity a secret, and this adds an extra layer of drama to their relationship in the fanfiction.
Common themes include the exploration of the vampire world. How does it function? What are the rules? And how do Sasuke and Hinata fit into it? Another theme is the protection of loved ones. Sasuke might want to protect Hinata from the dangers of the vampire world, or Hinata could be the one protecting Sasuke from other threats. And of course, there's the theme of sacrifice. They might have to sacrifice something in order to be together in this vampire - themed scenario.