Hello, and thanks for your question, with many helpful details.
If the expensive food from the vet is Science Diet, expensive yes, good for the kitties, not so much. Read the ingredients, and you'll see that it contains corn, gluten, meat by products and artificial additives/preservatives. This may be part of the problem, as these types of ingredients are allergens for cats.
Vomiting in cats can be caused by a variety of factors and allergies, hairballs and eating too quickly are all on top of the list. Acid reflux may also be a factor, so ask the vet if you can give them each 1/4 tablet Pepcid AC daily, or a few times a week, and see if this makes a positive difference.
Your Cornish Rex, who eats too quickly, I can help with instructions to slow her down. Raising up your cats' food/water dishes to chest level, should help, as this aids digestion. Place a new, unused ping-pong or golf ball in the middle of the bowl of dry food, to make her eat 'around' it. For canned food, serve on a flat plate, mashed down with a fork, so it's not 'mounded up' in the middle.
Products containing Slippery Elm may also be given to cats for digestive problems, and you can see more about such products, here:
I'm not advocating the use of any of these products, because I've never used them, but just including this for your information.
Giving a teaspoon of plain, unflavored yogurt daily, mixed into a small amount of canned food, will provide the 'probiotic' or 'good' bacteria, you'll read about in the site above. Most vets are not thrilled with 'holistic' products, unless they're holistic vets, but keep in mind that just because it's 'holistic' or 'herbal', doesn't always mean it's safe for your cat, and you should always check with your vet first, before giving any new or over the counter medication or product.
I hope you see an improvement, soon!