Whatever her reason for vomiting, she sounds like she's upset at bedtime. If she's that upset, she's going to have problems going to sleep. Can you sleep when you are really upset? You are trapped in a vicious circle. If bedtime becomes fraught with unpleasant associations, of course she's going to fight it!
I agree with the other posters that the best option is to work on why she is so upset about going to sleep. Some of the suggestions below are good, but she's also old enough to discuss it. Let me tell you a recent story.
My daughter (2 3/4 now) is an awful sleeper, so I don't consider myself an expert, but she's never fought sleep. A few months ago she started strongly resisting going to sleep. We'd lie down, and she would close her eyes and then pop up and tell us she did not want to go to sleep. I started getting really frustrated for a few days, then decided to ask her why. Turns out, she was having nightmares, where big red eyed monsters were chasing her, and she was afraid to fall asleep. We gave her an imaginary stick to beat off the monsters (my husband wanted to just have them fall asleep, but she preferred the more violent approach.) Voila- problem solved. She still isn't a great sleeper, but she happily lies down to try.
Some other ideas: We keep the door open to the hall so she has light, or she freaks out. Sometimes she just isn't ready to sleep, so I tell her she has to stay in bed, and that she can play with her animals or read a book. I check on her "in 15 minutes." Usually she pops up every 5 minutes or so, but I deal with whatever request and guide her back to bed. Normally after that 15 minutes she's ready to lie down and try to sleep again.
Good luck, and remember you are in this WITH your daughter, not against her.
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Alright, I normally don't do this, but I can't stop thinking about some of the advice here. PLEASE DO NOT LET HER SIT IN HER OWN VOMIT!
Look, there are two choices about why she's throwing up:
1) she's so upset that she's vomiting spontaneously.
2) she's so upset that she's willing to force herself to vomit in a last-ditch attempt to get your attention.
Either way, she's so upset that she's throwing up. I can't stand the idea that this is somehow "manipulative." Sure, maybe she's figured out that the only way to get mom to come in the room is to regurgitate. Think about how desperate you'd have to be to do that. Pretty awful, huh?
To now ignore her is to teach her that you just really don't care. No matter how bad she feels, no matter what she's willing to do. You don't care.
Sorry to be so harsh, but I keep running across this. Always in relation to crying it out. Never, hey, whenever my kid doesn't get what he wants he throws up, and stands there and smiles at me. That could be manipulative. This is not manipulative. This is a cry for help.