There are four bits to be aware of. I'll try and keep it spoiler-light, but it's difficult not to give some things away so here's your SPOILER WARNING.
- After Katie has been running around inside the walls eating scorpions, her parents get her back into her bedroom and force her to vomit with their fingers. It's one unbroken shot from a medium distance, clearly using a pump, but it's graphic and lasts a good 10 seconds or so. This is the worst one, but it's easy to see coming.
- When Katie's mom is screaming "Say something!" over and over again, Katie spits a large amount of fluid in her face. Not really vomit, but may trigger more sensitive people. It's the scene immediately after the mom and dad argue about whether or not to keep Katie in the house.
- At a wake, after Katie knocks the coffin over and starts licking up the embalming fluid from the floor, there's a shot of an old woman vomiting into her wine glass. Quite graphic, but quick and played for laughs. You could get away with just plugging your ears and eyes once the coffin drops.
- During the ritual, a stream of black goo is transferred twice from one person's mouth to another. This happens twice: once on the videotape the police officer shows the parents (grainy, from a distance, not particularly graphic), and once when the spirit is transferred to someone else towards the end of the film (more graphic, but it doesn't look or sound like vomiting - more supernatural.)
The first and third instances are worst for emetophobes, but they're both pretty easy to avoid. For the first one, if you want to be really safe, leave the auditorium once she eats a scorpion and come back in a minute or two - alternately, just block your eyes and ears when they're wrestling her back into her room. For the third, just block your ears and eyes when you see her licking the embalming fluid from the floor - if you want to be really safe and don't mind missing a highlight scene, leave the auditorium once Katie drops down onto the coffin for a minute or two.