Maybe in the fanfic, Vegeta did something unforgivable like betraying Bulma's trust or putting her in great danger while under the influence of the Majin transformation. For example, if he endangered their family or made a pact with the enemy that directly opposed Bulma's values.
Bulma might cut off all communication with Vegeta. She could focus solely on her own work and taking care of their child without Vegeta's presence. For instance, she might pour all her energy into her scientific research.
Maybe in the fanfic, Majin Boo has done something extremely unforgivable to Bulma, like harming her family or friends. This could be the reason for her inability to forgive.
Maybe Bulma is too busy with her scientific research and family matters to be interested in fanfic about Vegeta. She has a lot on her plate, from taking care of Trunks to working on new inventions.
It could also be due to the general nature of Majin Boo. He was initially a villain created for evil purposes. His appearance and the way he carried out his actions were often grotesque and malevolent. Bulma, who is used to a more civilized and scientific environment, might be repulsed by his very existence and find it unforgivable that he was able to cause so much trouble in their world.
One possible source of angst in such a fanfic could be the internal struggle of Majin Vegeta. He has this new, dark power that might put his relationship with Bulma at risk. For example, his altered state of mind due to the Majin influence could lead to him pushing Bulma away, fearing he'll harm her.
The relationship in the 'Majin Vegeta x Bulma Fanfic' can be quite interesting. Majin Vegeta is a character with a new set of emotions and powers. Bulma, on the other hand, is a smart and strong - willed woman. The fanfic may depict how their relationship evolves in the face of challenges, like Vegeta's inner struggle with his new form and Bulma's attempts to understand him. It might also show how their love endures or changes during this unique situation.
Bulma's inability to forgive could also drive her to take actions on her own. Maybe she tries to find ways to keep Majin Boo at bay or even eliminate him, which could lead to new adventures and challenges. It could also lead to her developing new powers or technologies in order to deal with the threat she perceives from Majin Boo, which would significantly impact the storyline.
It could be that Majin Boo attacked the city where Bulma lives. Her home and the people she cares about are in danger because of him, and that's why she can't forgive him.
The plot could start with Majin Vegeta's pride getting in the way. He might feel that Bulma is disrespecting his new power as Majin Vegeta. So, he challenges her to a fight. Bulma initially panics but then decides to stand her ground. She uses her knowledge of Saiyan physiology (from all the time she's spent with Vegeta and the other Saiyans) to find his weak points. However, she also tries to reach out to the real Vegeta inside. As the fight progresses, Vegeta starts to see how strong - willed Bulma is and slowly begins to break free from the Majin influence.