It often stems from fans' imagination and their love for these characters. They might envision unique scenarios and storylines that bring Bulma and Vegeta together in different ways.
The Bulma x Vegeta fanfiction usually happens because fans are passionate about their potential relationship. They might draw inspiration from little hints or moments in the original series and expand on them to create elaborate stories.
In the fanfiction, their relationship could be explored by showing how they adapt to each other. Vegeta's feral side might require Bulma to be more patient and understanding. And Bulma's needs might force Vegeta to become more domesticated in some ways. For instance, he might have to learn to control his temper around her family and friends. It could also show how their relationship evolves over time, starting from initial attraction despite their differences to a deep, loving bond that endures through various challenges.
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.
In many fanfictions, their love starts with unexpected encounters and shared experiences that bring them closer.
In such fanfictions, there could be various scenarios. It could be that someone hurt Bulma physically or emotionally, and Vegeta is out for revenge. Or perhaps there's a mystery to solve about what happened to her. The possibilities are endless!
In such fanfiction, Vegeta might make Bulma happy by showing his softer side. Maybe he cooks a special meal for her. Vegeta is often portrayed as tough, but when he does something out of his usual character like cooking, it can be a sweet surprise for Bulma.
Vegeta would likely still be his proud and strong - willed self, but with a newfound tenderness towards Bulma. He might be a bit more protective and less quick to anger. Bulma, on the other hand, would be her intelligent and sassy self, always ready to challenge Vegeta in a playful way.
Another character could be Dr. Briefs, Bulma's father. In the alternate universe, he might have a more prominent role in bringing Vegeta and Bulma together. For example, he could be the one who arranges for Vegeta to work on a project with Bulma, which starts their relationship on a more professional note that gradually turns into something more personal.
The relationship might develop gradually. Bulma could start by being attracted to Vegeta's power and mystery. As she gets closer to him, she discovers his more complex personality. Vegeta may initially resist her advances but eventually gives in to his own feelings.
Their relationship would likely become more intimate and committed. With a baby on the way, Vegeta would probably be more protective of Bulma. He might show his love in more practical ways, like making sure she has everything she needs.
At first, it might be a relationship based on mutual need. Vegeta needs Bulma's resources and intelligence to get back on his feet, and Bulma is intrigued by this alien. As time passes, they start to understand each other better. For example, Vegeta may start to open up about his past and his Saiyan pride, and Bulma will share her dreams and ambitions with him.