We used a whoopie cushion (of all things!) attached to one of those rubber hoses a bit like the ones you find on bunsen burners in high school chemistry labs. I seem to remember there being a convenient point where the actress is offstage not too long before the vomiting, and we strapped it (carefully!) to her then, with the cushion, filled with fluid, under her arm, and the top of the tube just in the neck of her costume. At the appropriate moment, she'd squeeze the cushion against her body with her arm, and it would force the liquid up through the tube. I think we may have used a hose clip to join the cushion and hose, that way it could be detached to allow easier filling up of the cushion. Not sure what we used as the vomit - something not too thick! I wasn't actually working on the show, just floating on the periphery - I'll get hold of the SM and see if she can add any more detail!