Why Is My Dog Vomiting Bright Yellow Stuff?

He'll be fine. Make sure he has plenty of water. Takes awhile for what ever is ailing him, to pass through the system. He is an animal, not a human so sick dogs don't eat when they are sick so there is nothing to pass. They instinctively know how to take care of themselves. Give him a few days of not eating, and then it'll pass though his system and he'll get  his appetite back. Just keep water and food on hand. If it is more then a few days, seek a vet.

Yellow/white/foamy vomiting in dogs if occurs of and on then no need to worry bit persistent yellow vomiting in dogs can be due to many reasons like

1. Canine bloat
2. Food allergy
3. Eating rapidly
   
4. Overeating
5. Nervousness
6. Stress
7. Dry foods

Among above mentioned reason, bloat is more dangerous because death is possible if not treated on time. So, take your dog to vet for diagnosis and treatment.

It might be because of the food he eats.Or it could be because of something he ate that he wasn't supposed to.

It sounds as though this could be viral, try and take some samples to the vets asap