Maybe it's because Bulma has her own established character traits and relationships in the original story. She might not fit well into the scenarios often presented in Vegito fanfic. Also, her character is more focused on other aspects like her scientific work and family in the canon, so fanfic ideas involving her in a Vegito - related context could seem out of place to her character.
Bulma could reject Vegito fanfic because the pairing or the concepts in those fanfics might go against her character's integrity. In the official story, she has a particular role and personality. Fanfics often play around with different pairings and ideas that might not align with how she is portrayed. For example, she is more associated with Yamcha and later Vegeta in the official story, not with the fusion of Goku and Vegeta which is Vegito.
One reason could be that the fanfic might distort her character. Bulma is a smart scientist, and if the Vegito fanfic makes her act in a way that's not true to her intelligent and rational self, she would seem to reject it. For instance, if the fanfic has her being overly emotional or doing things that are completely against her scientific nature just to fit into a Vegito - centered plot.
Well, it could be that Bulma realizes that the relationship with Vegito is just too complicated or dangerous. Maybe she decides that she can't handle the chaos that often surrounds Vegito, like all the battles and threats that come with being associated with him.
To write a Bulma leaves Vegito fanfic, start with character development. Understand Bulma's and Vegito's personalities well. Maybe Bulma has grown tired of Vegito's constant battles and wants a more peaceful life. Then, build a conflict that leads to her leaving. As you write, use vivid descriptions to bring the story to life. You can include flashbacks to show the history of their relationship. And make sure to have a resolution at the end, whether it's a reconciliation or a final goodbye.
It could be Frieza. He might be jealous of Vegito's power and decides to target Bulma to get to Vegito. His actions could make Bulma realize the danger she's in because of her association with Vegito, leading her to give up on him.
You might try fanfic websites like Archive of Our Own or FanFiction.net. Just search for the relevant characters in the search bar on those sites.
Honestly, I haven't come across a fanfic that specifically features Bulma, this 'have him' idea, Chi - Chi and Vegito in a great way that I could recommend. However, when exploring fanfic sites, pay attention to the tags. If a fanfic is tagged with all or most of these characters, it's more likely to be relevant. Also, look for writers who are known for their well - written Dragon Ball fanfics. Sometimes they might not have exactly what you're looking for, but their writing style could still be enjoyable and they might create something similar in the future.
There's a fanfic called 'Breaking Ties' where Bulma leaves Vegito. This fanfic has a unique take on their relationship and the reasons for her departure. It shows the complexity of their relationship. Some other fanfics in this category are 'Beyond the Split' which looks at the long - term effects of their separation, and 'A Solitary Choice' which focuses on Bulma's internal struggle before making the choice to leave Vegito.
One common theme is power comparison. Writers often explore how Vegito's power from Dragon Ball stacks up against the powers in the other crossover universe, like the magic in a Harry Potter crossover or the quirks in a My Hero Academia crossover.
Yamcha could potentially betray Bulma. Even though they have a history together, in a fanfic scenario, he could be jealous of Bulma's growing closeness with Vegeta or her scientific achievements. So, he might do something to undermine her, like spreading false rumors about her work.
In such fanfics, Bulma might be portrayed as more adventurous in her love life. She could be shown as being open to new relationships outside of her usual circle, which might bring out different aspects of her personality, like her boldness or her ability to adapt to new situations.