Perhaps Spider - Man starts to train harder in order to be better prepared for future threats. He could seek out new mentors, like Doctor Strange if he's still around. He might also get involved in some sort of community rebuilding effort, using his powers to help the common people who are now in a state of chaos and fear.
One idea could be that Peter Parker decides to unmask himself as a political statement during the Civil War. He might feel that the secrecy in the superhero community is causing more harm than good and by unmasking, he hopes to start a new era of transparency among heroes.
Well, if Spider - Man died in a Civil War fanfiction, it could be that his death serves as a catalyst for the end of the war. His death might make both sides realize how pointless their fighting is. Maybe the heroes would then come together to honor his memory and try to make amends. His friends in the superhero community, like Ned if he knew about Peter's identity, would be heartbroken. And villains might take advantage of the situation, thinking the heroes are weakened without Spider - Man, which could lead to new threats emerging in the fanfiction world.
Spider-Man comics typically focus on his own adventures and villains in New York. I haven't come across any instances where the Vietnam War was specifically mentioned. Maybe in some obscure alternate universe storylines, but not in the mainstream ones.
Spider-Man's age in the Civil War comics varies depending on the specific storyline and interpretation. Generally, though, he's around 18-21 years old.
Spider-Man doesn't meet his demise in the Infinity War comics. The plot unfolds in a way that ensures his presence throughout and beyond this particular storyline.