I agree 100%. You need to get a diagnosis for this illness. Vomiting with diarrhea and lethargy are not normal for a simple change in diet. If he can't keep water down, that is an even bigger medical emergency.
Is your dog still feeling sick? If so, I would take him/her to the vet to see what's going on. I would also go back to Iams for now. It might be totally unrelated to the food switch...maybe she ate someting outside that made her sick, for example...but until you see the vet, I wouldn't take a chance.
I hope your dog feels better soon!