It's a bit hard to say for sure. Both Vegeta and Goku have their moments of strength. Goku has some unique powers and techniques that give him an edge at times, but Vegeta is no slouch either and constantly works to catch up and surpass Goku.
Generally, Goku still holds the edge over Vegeta. Their power levels fluctuate, but Goku often achieves higher peaks of strength and unlocks more powerful forms first.
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.
Well, if they were brothers in a fanfic, it could change how they interact with the other characters. Maybe they'd have a deeper understanding and fewer power struggles between them. Also, their family background and upbringing could be explored in new and interesting ways.
It often starts with a shock or surprise at the mpreg situation. Then, they gradually learn to rely on each other more. For example, Vegeta might start to show a more caring side that he usually hides, and Goku might become more responsible. They'll have to communicate more, sharing their fears and hopes about the future with the baby.