In many fanfics, it's really hard to say for sure. Android 18 has her android strength and durability. Vegeta, on the other hand, is a powerful Saiyan with his Super Saiyan forms. If the fanfic writer focuses on Android 18's infinite energy, she could potentially outlast Vegeta. But if Vegeta quickly ascends to a high - level Super Saiyan form like Super Saiyan Blue, his power could be overwhelming.
A 'bardock vs king vegeta fanfic' could explore the backstories of both characters. Bardock's story as a Saiyan who starts to question the Frieza regime might contrast with King Vegeta's more traditional and perhaps self - serving rule. Their fight could be a symbol of the changing tides within the Saiyan community. It could also introduce new elements to the Dragon Ball lore, like different forms of Saiyan power that are unique to this fictional encounter.
In a fanfic, Naruto might power up by having a spiritual awakening. He could connect with the spirits of past Hokages in a deeper way, gaining their knowledge and some of their power. This could enhance his existing jutsu and give him new abilities. Vegeta could power up if he were to face a near - death experience against Naruto. This could trigger a hidden potential within him, like a new type of Super Saiyan form that has unique abilities. He could also fuse with another Saiyan (if there were another Saiyan available in the fanfic's scenario) to create an even more powerful being.
In the fanfic, Bardock might be filled with pride. He could be thinking how his son has grown into such a powerful warrior. After all, Goku has come a long way from being a little Saiyan baby sent to Earth. Watching Goku fight Buu, Bardock might see all the potential he always knew Goku had, finally being fully unleashed.
In the fanfiction, Bardock might be in a state of awe. Goku's fight against Buu is no small feat. Bardock would likely note Goku's ability to keep getting back up, no matter how hard Buu hits him. He might also be thinking about the power levels involved. In his time, the power levels he knew were much lower. Now, seeing Goku take on a foe as powerful as Buu, he'd realize just how much the Saiyan race has evolved, especially with Goku as a prime example.
Another theme could be loyalty. Bardock might be loyal to the true potential of the Saiyan race while King Vegeta is loyal to the existing power structure. This difference in loyalty can create a great conflict in the fanfic. Also, there could be a theme of social change. As Bardock may represent the underclass Saiyans who want a change, and King Vegeta is the symbol of the old regime.