My protagonist is vain and pretentious (though actually insecure), and has quite a cruel personality. How would I go about writing her in a way that would not result in the readers hating her? Sorry if this is too vague.
Most people, and therefore most characters, have both good and bad traits. Granted, some people have more or less of one than the other, but even the worst villain in fiction should have some redeeming qualities, assuming the villain is human.
Because this is your protagonist, you should try even harder to give her some qualities that make her flaws bearable. Her insecurities might help readers feel some sympathy for her, but it won’t quite do enough to make her a likable character.
Something that I think might help you, and something I think many writers overlook, is this: A character can be disliked by other characters and still be liked by readers.
The protagonist isn’t only special because the story centers around them; they’re special because they are one of the characters that the readers get to know on a deep, intimate level. The readers get to see a side of the protagonist that no one else gets to see. So while most people view her as vain and pretentious, and cruel, the readers might see that she’s also creative, smart, and independent.
So let’s recap before I move onto the next point. You have a protagonist who you’ve shown to be cruel to other people, and she appears to take great pride in her appearance, maybe even a little boastful about the things she owns and her perceived superiority. And then, we see something about her life that other people don’t see, and in this it’s revealed that she puts on a facade of pseudo self confidence to cover how she really feels about herself.
Then we might see the bills she’s paying on her own, the painting she’s struggling to complete, and maybe even the roommate that she’s actually super nice to. This is where you show some of her more redeemable qualities, and you attempt to explain the source of her cruel attitude.
Another way you redeem a character’s less-than-desirable qualities is to show how the events of the story help her improve on them. It could be that something happens to her that makes her realize that she doesn’t need to put on an act to hide her insecurities, or even better, she has an unsettling moment where she sees how her cruelty has affected someone who isn’t that different from her. And then (the best part) she does something to make it right.
We admire people that can recognize their flaws and attempt to change them. So your character might start off as vain and pretentious and cruel, but she doesn’t have to stay that way. Allow the story events to change her, because readers expect that, even in characters that start off as well-liked, kind-hearted people.
She can be cruel, but make her more than that. Give her some depth. Show us that there’s more to her than what most people see of her on the surface.