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.
Another theme could be the clash of different universes. Amazo is from the DC universe and Saitama is from One Punch Man. Fanfics might play around with how these two different universes' rules and powers interact when these two characters meet. Also, there could be themes related to respect and rivalry. Saitama is often very nonchalant about his power, while Amazo is a complex android with various abilities. How they view each other in the fanfic can create interesting storylines.
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.
Saitama would win. He has shown incredible strength with just one punch. His power seems limitless, and in a fanfiction scenario, his simplicity and raw power would likely overpower Jiren's more complex abilities.
One possible plot twist could be that Jiren discovers a way to nullify Saitama's strength. Maybe he finds some sort of energy - draining technique that weakens Saitama over time. Another could be that Saitama holds back for some reason, like he wants to see the full extent of Jiren's power, and in doing so, he gets into a real struggle.
Saitama-sensei didn't have a mosquito fight in One Punch Superman because Saitama-sensei was a fictional character whose story and plot were adapted from manga and anime. In the manga and anime, Saitama-sensei did not fight mosquitoes. He won the game by relying on his strength and skills rather than relying on the help of mosquitoes. If you want to know more about Saitama-sensei, you can search for related manga and animation works.