All Nikita's tests came back normal, so the only X factor in this equation is Dick Van Patten's Unnatural Offbalance food.
The vet thinks that must have caused the REPEATED PROJECTILE VOMITING and EXTREME DIGESTIVE TRACT UPSET.
Just a little bit of the new food mixed in with her regular food was enough to do the trick. So, lesson learned - no new food, ever - stick with the one that works (which is Wellness, btw).
I'd never have guessed that something purported to be so high-quality would cause such a problem. I'd like to punch Dick Van Patten in his big fat pasty face.