Jeff's ideas are because of his Percival bias.
Explanation:
Jeff has a predisposition bias against vegans because he associates them with a certain ind of habits that is the tree hugging and many such movements that he obviously has a negative view about.
Thus, even when a person is vegan because they have no other choice but to be one they are still laying the same negative stereotypes on them.
This is the predisposed bias that he transfers onto another person who broadly fits that category in his head.