I want to ask my bulimic roommate to control herself
It's unfortunate she is struggling with bulimia, but can you explain to me why you would approach her in this manner?
Because she makes me sick to watch her vomit all the time.
I can definitely understand how it would not be pleasant, but it's important that you take care and consideration when you talk to her. Try to be supportive and offer to assist her in seeking the treatment that she crucially needs and deserves.
I hope that works.
Let her know in a calm, gentle way how her bulimia is impacting her health.