One idea could be that Vampire Sasuke is forced to wear the orange sweater as a form of punishment by an evil wizard. The orange color might be a magical hue that suppresses his vampire powers, leading to all sorts of comical and challenging situations as he tries to break free from its effects.
Imagine a grand vampire ball. All the vampires are dressed in elegant black and red attire. Then there's Sasuke, forced to enter in the ugliest orange sweater. The other vampires snicker and whisper as he walks in. Sasuke, with his pride as a vampire, holds his head high, but inside he's seething. He plans his revenge on whoever put him in this hideous garment. As the night progresses, the sweater starts to have a strange effect on him, making him more visible to the sunlight, adding to his troubles.
I'm not sure there are a whole lot of fanfictions specifically about 'Vampire Sasuke' and an 'Ugliest Sweater'. However, you could start by checking out some of the major Naruto fanfiction websites. Maybe search for Sasuke in a vampire - like AU (alternate universe) and see if there are any mentions of an ugly sweater in those stories.
Here's another idea. There is a powerful vampire council that enforces certain rules. Sasuke, as a vampire, breaks one of the rules and as a punishment, he is forced to wear the ugliest sweater known to the vampire world. This sweater makes him stand out and he becomes a target for vampire hunters. He has to find a way to remove the sweater and clear his name with the council while avoiding the hunters.
One plot idea could be that Sasuke is a vampire who has been secretly protecting Sakura from other dark forces in the vampire world. Another could be that Sakura discovers she has a special blood type that is very alluring to vampires, and Sasuke has to fight his instincts to protect her. Maybe they are both turned into vampires and have to navigate their new existence together.
One idea could be that Sasuke, a vampire, is initially drawn to Hinata because of her unique chakra. Hinata, on the other hand, is intrigued by Sasuke's mysterious allure. Maybe Sasuke tries to resist his vampire instincts around her, which creates a lot of internal conflict for him.
One plot idea could be that Sasuke, as a vampire, is initially conflicted about his feelings for Sakura. He might resist his attraction to her because he doesn't want to bring her into his dangerous vampire world. Another idea is that Sakura discovers Sasuke's vampire nature and tries to find a way to cure him, while Sasuke doesn't want to be cured as he sees his vampirism as part of his power. A third idea could be that there is a vampire clan war and Sasuke has to protect Sakura from the other vampires who see her as a weakness of Sasuke's.
A plot might involve Sasuke's transformation into a vampire being a result of some dark jutsu experiment gone wrong. Naruto, who is determined to save his friend, embarks on a journey to find a way to reverse it. Along the way, they encounter various characters from the Naruto universe who either help or hinder their progress. There could be a sub - plot where Sasuke struggles with his new vampire instincts, especially when it comes to his feelings for Naruto, as vampires in this fanfiction might have heightened emotions.
One plot idea could be that Sasuke is a werewolf and Sakura is a vampire. They are from rival clans but fall in love. Their love has to overcome the prejudice and enmity between their kinds. For example, they might have to hide their relationship from their respective families and communities who would be against it.
One idea could be that Sasuke is a half - vampire who tries to hide his identity at high school. Sakura, being a curious girl, starts to suspect something strange about him. She could then discover his secret and they end up having to work together to protect the school from other more dangerous vampires.
One common theme is the struggle between Sasuke's vampire nature and his humanity. He often has to deal with his bloodlust while still trying to hold on to his old self. Another theme is the isolation that comes with being a vampire. Sasuke is usually depicted as being alone or having a hard time fitting in with normal humans or even other vampires.