There are several common plots. One is the idea of Vegeta having to grow up faster in some ways to match Bulma's maturity level. This can lead to his character growth in unexpected ways. Another plot could be centered around how others in the Dragon Ball world react to their relationship. Some might be skeptical or disapproving because of the age difference, which then becomes a source of conflict in the fanfiction. Additionally, there could be plots about them building a family with the age difference playing a role in how they raise their children.
One common plot is the exploration of their relationship development. With Vegeta being younger, there might be hesitations from Bulma's side at first. Another plot could be Vegeta trying to prove himself worthy despite his younger age, especially in terms of his power compared to others in the Dragon Ball universe.
Well, a frequent plot is how their different life experiences due to the age gap affect their interactions. For example, Bulma may have more knowledge about the world and technology, while Vegeta is focused on his Saiyan training. Also, some fanfictions might have a plot where they face challenges together, and the age difference either helps or hinders them in dealing with those challenges. It can create interesting conflicts and resolutions within the story.