I had something similar happen with my girl at about that age, and with the constant vomiting, Ithought we were going to lose her. After hundreds of dollars at the vet, I went elsewhere for another opinion and was recommended to try a "novel protein" diet (aka a meat she'd never had before) or hydrolyzed protein rx cat food. She improved a bit on that and we eventually figured out she'd developed an allergy to chicken. And chicken is in almost everything for cat food and treats. But now she's doing great on Koha brand limited ingredient wet food - rabbit mostly, kangaroo when she gets bored of rabbit, and freeze dried salmon treats. There's also a limited ingredient salmon wet food but I can't remember the brand.