Shelley I’m guessing Stevie isn’t used to using that pronoun and also she probably refers to Vi as her because she wasn’t informed directly by Vi to the correct pronoun. Maybe Stevie didn’t want to assume on second hand information on Vi’s preferred gender pronoun.
Hilde I'm super late but like, this bothered me too! The other answer to this is wrong btw! Stevie continues to use she/her pronouns for Vi even AFTER she was corrected on this by her friend Janelle... This made me dislike Stevie but mostly the author for doing this, and not even explaining it! I don't even understand how this got trough editing.
Lukyan's Library Since it's from Stevie's perspective, she referred to Vi as she/her because Stevie was uneducated on Vi's pronouns, which Jannelle explains to her later. From then on, Vi is referred to as they/them.