I'm no expert on 2nd edition, but I would assume either a straight Constitution check, or a save vs. poison? Sorta depends on what the cause of the vomiting is. If it's from breathing, drinking, or eating something that would induce vomiting, that sounds like a save vs. poison. If it's enduring g-forces, having an actual disease, suffering from morning sickness(?), or being slammed in the stomach? Sounds like a Constitution check, to keep your body from doing a reflexive action you don't want it to do.