A while ago, I started writing a short story for a competition. It was supposed to be about four girls in a shared student apartment. Plan was to have everyone conflict with everyone until they united against a common enemy (the landlord or the exams - I haven't reached that far), and learnt to put aside their differences.

I soon ran into a problem: one of the girls, a vampire, had much more oomph than the others. She was in conflict with the werewolf girl because werewolf, she was in conflict with the Ifrit girl because the latter belonged to the nationality whom the former blamed for being dead, she was in conflict with the religious vanilla POV girl because a religious issue is how she ended up a vampire instead of being dead-dead. Most of all, she was in conflict with herself, because dying and waking up a vampire really threw a wrench into the plans she had for her life. She was a walking-talking explosion.

The other girls had their conflicts, but those conflicts were mundane. Vampire - she was boiling over, which caused her to dominate every scene, and every scene without her became drab by comparison.

I tried to amplify the others' existential anger, but then there was so much conflict that the story got mired down in arguments, and wouldn't progress. I tried to make it Vampire's story and get rid of the others, but that didn't work either - she needed the other girls to bounce off, they allowed her to let out some penned-up rage. They "represent" in a way the people/bodies/situation she's angry with.

How can I address such a situation, when one character in a group is drastically overshadowing the others? Please note I am interested in general approach answers, in the spirit of the two examples I presented in the previous paragraph. I am not looking for specific "write this" answers - those are off topic, and wouldn't help anybody else, nor me when I run into such a character again.

If your givens aren't working, change your givens.

If Vampirella McExplosia is dominating every scene she's in, then she's too big for this story. Save this draft (so you aren't putting a stake in her, just moving her) and rewrite your story entirely with someone else as Roommate #4.

You will have to change plot points, and probably the entire main conflict of the story, but it will solve your problem. She's a great character who's the wrong fit for this work.

See also
My cool character is doing nothing for the plot. How do I deal with him?

Introduce the actual plot?

You may or may not actually have a problem. You have a character who is inherently more interesting than the others, she will naturally dominate the part of the story focussing on character introductions and set up.

One solution is indeed to balance the characters but you can also just move forward and introduce plot(s) that directly involves the other characters. If it is interesting the characters will be more interesting and the story will balance out.

If you naturally started with one character dominating, the easy way would be to keep it up by splitting the story to four parts each of which is dominated by a single character. A spotlight approach where each target is illuminated in its own turn.

Naturally you may also have on actual problem and need to really balance the characters but other answers will almost certainly cover that well enough so I won't.

My approach (which I have taken) would be to abandon that story, think much more about Cindy the Vampire, and write a story in which she is the sole hero, and her anger and explosions end up having some positive effect on the world, and she comes to terms with the loss of her old life, and embraces her new life, anger and all. She just learns to channel them into something good. "Angel" the spin-off of Buffy the Vampire Slayer, did something similar.

I don't suggest rewriting Angel, I'm just noting that the concept isn't too far out.

You have a strong character! Don't throw her out, give her a job.

To me, one of the more difficult things about writing is finding a strong character that isn't just a flawless Superman (or Superwoman). Your vampire sounds good, she creates conflict, she is special, and she is flawed (angry with pent-up rage). Give her some hints of an underlying sensitivity and altruism, which we will want to come to the fore eventually, and you have a character transformation story.

This can be a Young Adult novel, coming-of-age, or a New Adult novel (for 18-28 ages) that are generally also a kind of coming-of-age story, just about an age range where people are out of high school) and independent but still trying to figure out who they are and what they do in the world, romantically and/or professionally.

You have a good character. Find her weaknesses, find her strengths, find the good in her, even if it has been buried in her rage. What does she DO in a day? Is it satisfying, or does she hate it? What would she rather be doing? Is it just restless energy, or is it driven by some underlying passion? (I would) put aside the short story, that was just a stepping stone to finding this character. Don't lock her back in the coffin. Embrace her, and you have the titular character of a novel, or even a series, The Adventures of Cindy the Vampire.

I recommend reading Refund High School. One of the major characters, Mari, is this to a tee. (Just not a vampire... yet...) We see her start off as a strong and loud (bratty) character, but she's tolerable because we only see her for moments. As the story progresses, she becomes a more like-able and well-rounded character. She is still the bolstrous brat spitfire she started as, but her energies have since been redirected in a way to improve upon herself and her life. I feel like you could get some inspiration from here in order to push your Vamp through to being not TOO much, but (rather) just enough.