A dog may vomit yellow foam simply because his stomach is empty and the bile can be irritating. If your dog is otherwise healthy — and he's eating and defecating normally — it may help to reduce the time in between meals. But this doesn’t mean that your dog should be fed more. Instead, one meal can be divided into two or three smaller but more frequent feedings. Unfortunately, there are many medical conditions that can cause a dog to vomit on an empty stomach, so a phone call to your vet is a good idea if you notice this problem. Potential Medical Concerns If there’s something in the vomit besides yellow foam, it may help to bring a sample to your veterinarian.