Vernon could be quite curious. He'll be eager to read what others have come up with. There might be new ideas, different takes on his character or his world that he hadn't thought of before. So he's probably excited to explore these new interpretations.
He could be in shock at first. Just standing there, unable to believe what has happened.
He probably feels extremely hurt and confused. Betrayal by friends is a very painful experience, and Bell might question his own judgment in trusting them in the first place.
Perhaps Vernon is a character himself in a particular fandom. In that case, 'Vernon gets his fanfiction' might mean that the creators or fans within that fandom have written stories specifically about Vernon. It could be a really exciting moment for Vernon if he's aware of it, as it shows his popularity within the community.
If his friends continue to ignore him, Harry could become sad and withdrawn himself. He might start to isolate more, spending time alone in places like the Forbidden Forest or an empty classroom. He might also lose his confidence in himself, thinking that he is not worthy of their friendship. In his loneliness, he could focus more on his studies or some solitary magical pursuits, trying to distract himself from the pain of being ignored.
His life could change drastically. He might start to see the magical world in a different light earlier than in the original. For example, he could meet some early allies or enemies right after getting the letter.
One possible reason could be his performance at work. Maybe he repeatedly failed to meet his targets or made too many mistakes. For example, if he was in a sales job and couldn't close enough deals, his boss might decide to let him go.
He feels a great deal of sadness. He was not given a fair chance at life.
He might feel excited. Rollercoasters are thrilling, and having a friend by his side could add to the fun.
He might be initially shocked. Harry has always trusted his friends, so when the betrayal happens, it would be like a bolt from the blue. He could stand there speechless for a moment, unable to believe what has just occurred.
He may feel a bit weird. Fanfiction can be so different from his real self. He might not like some of the wild ideas people have about him.