It sounds like you and your cat are going through quite an ordeal!
Many things can cause cats to vomit. Food intolerance, metabolic disorders (such as liver or kidney disease), foreign objects in the stomach, neurological problems, heartworm disease, intestinal parasites, tumors, problems with the pancreas and exposure to poisons are among the many offenders, and this list is far from complete.
However, in cats that have a long history of vomiting that worsens precipitously I am often suspicious of a syndrome called inflammatory (or infiltrative) bowel disease. The syndrome is also known as IBD.
Cats with IBD suffer chronic irritation of the intestines. This leads to vomiting, diarrhea, or both.
Diagnosing IBD can be difficult. A biopsy of the intestines (by means of surgery or endoscopy) is the most definitive method, but it is very invasive and I rarely recommend it.
However, you do not mention that your cat has had an ultrasound of her abdomen. Ultrasound is a powerful, non-invasive and painless diagnostic technique. Skilled ultrasonographers can sometimes diagnose IBD. And they almost always offer insight into the cause of chronic vomiting. I would recommend abdominal ultrasound as the next step for your cat.
If your cat is diagnosed with IBD, several treatment options exist. Dietary modification is the mainstay of treatment, but it sounds like you may have exhausted this possibility. Probiotics help many cats; yours, unfortunately, seems refractory to this treatment.
I have seen many cats with chronic vomiting respond favorably to treatment with medicines that modulate the immune system. Prednisone is most commonly used. As I have mentioned before on this blog, prednisone is a double-edged sword. However, you may want to discuss a trial of the medicine with your veterinarian. It may help solve the problem.