Chronic vomiting with no obvious answers

Hi all,

I've been having some problems with my dog vomiting over the past two months or so on an irregular basis and I can't seem to figure it out. I have taken her to the vet and they have put her on a "treatment plan" but I really don't feel like they're treating her for the right thing.

The back story:

I got Josie as a rescue at almost 9 months old. I had her for about a month eating a good quality puppy food. I would go to my family's house (they have two dogs there) and she would eat faster there and occasionally throw up her food. We assumed this was because she was eating too fast as she wanted to get out and play with the other dogs. This happened for the next couple times we visited my family. After a couple more weeks though she started getting sick around my house- she's the only dog here and it's just her and I so I know exactly what she's being fed. Every time she would get sick it would be shortly after she ate and she would vomit up undigested food. (It is vomiting, not regurgitation). I would take her off the food and put her on rice & chicken or chicken broth and she'd be ok. After a few days I would mix in her dry dog food and she was still ok. After she had only dry dog food though she wouldn't want to eat it and if I left it down and didn't give her anything else she would eat it but then would vomit it again. I would put her back on rice & chicken broth and then over a week slowly transition to dry food. At this point I had had her for a couple months and was thinking maybe she was allergic to her food? I switched her to a grain-free food and she was fine for a week, no committing at all. I thought that must be it! Then after 8 days on the new food she vomited again. I called the vet at this point and said something was wrong and I couldn't figure out what it was. I took her in and they did a full exam. They didn't see any obvious things and decided to put her on a treatment plan. First they would treat her for parasites for 5 days to see if she had any (I guess doing the test is more expensive than the treatment itself so they just wanted to do the treatment). If that didn't resolve it then we would put her on prilosec to see if she has a reflux problem for a month. If that doesn't work then the next step would be to do a food study to determine if she is allergic to something.

So I took her home and gave her the de-worming powder medication every night. The vet told me to put her on her regular food so I did. Every time she ate she would have either wet dog food with her dry food or rice with her dog food (that I had left over from after she got sick before I took her to the vet). When the 5 days were over I gave her just the dry dog food. She reluctantly ate it . Dinner time is at 6pm and then at 3am I woke up to her vomiting. I gave her dry dog food again the next morning at 6am and within 30 mins she had thrown that up too. So obviously it wasn't the worms or other parasites that were a problem.

Here's my question- is there any kind of sickness where dogs get sick if they only eat dry dog food? She has NEVER thrown up when shes had anything else in her food- be that rice or wet dog food. It's only ever with plain dry dog food. She's had a number of different kinds of food inlucding two kinds of grain-free. The vet wants me to put her on prilosec now for a month or two to see if that helps but I don't wnat to do that if that's not the problem.

Has anyone heard of anything like this? Is this a real thing? Any recommendations?