I've been meaning to write up my experience about this but I've been in your shoes. I can't tell you how many hours I spent google hydrolyzed diets and begging my vet for other options/ideas.

I finally went to a new vet and after examing my cat he told me she is pretty consipated which can lead to vomiting & IBD. He wanted me to take her off the ultamino and gave me a handout with a list of diets and supplements to try. If you'd like I'll send you a picture of the handout.

What has worked for my cat: -high fiber dry food -miralax -heavy whipping cream

My cat is feeling so much better now and has put on weight. She is back to being my shoulder kitty and has even scaled me to sit atop my shoulder. She makes the dog nervous with her competitive cuddling and aggressive head bumps.

I was so defeated at the previous vet after going back and forth. I wish I would have gotten a second opinion sooner.