This is just my opinion, not telling anyone here what they should do, just what I'd do if it were my dog.
I'd test for parasites if not already done if my dog had vomiting or reflux.
99% of GERD is caused by food intolerance, or intolerance to an ingredient in a supplement. I've never been a big fan of Missing Link, and I am very careful to choose oil supplements very carefully. No, not careful, I'm actually paranoid about rancid oils. I think it's a huge problem in supplements for humans and pets. So many products that have fatty acids/oils in them have rancid oils, which can cause all sorts of things. Look at the ingredients in Missing Link:
Flaxseed, Rice Bran, Primary Dried Yeast, Cane Molasses, Sunflower Seed, Freeze Dried Beef Liver, Dehydrated Alfalfa Meal, Dried Carrot, Ground Beef Bone*, Dried Fish Solubles, Ground Barley Grass, Dried Kelp, Freeze Dried Oyster
Why molasses? Too many unknowns. Like, what is dried fish solubles? Where are they getting the other ingredients?
If it were my dog who had reflux issues, I'd try feeding only one protein, like ground bison, and one starchy veg or tuber, like sweet potato or pumpkin for a 3 weeks to a month, nothing else, no supplements, nothing, except maybe Animal Essentials Calcium, then see what happens. Just my opinion. Not telling anyone what to do, just what I'd do.