Given your dog's age and history I would be highly suspicious of a gastric foreign body, "garbage gut," toxin exposure, or pancreatitis
Gastric foreign bodies can cause an obstruction of the gastro-intestinal (GI) tract and can cause vomiting. This problem can be an emergency and often requires surgery to remove the foreign body. Common things we remove from the intestines of dogs include chew-toys, undigested rawhide, sticks and even underwear.
"Garbage gut" is a condition caused by eating inappropriate or rotten food items. WIthout causing an obstruction these things can cause irritation to the stomach
or intestines and can cause severe vomiting. Although this often can clear-up on its own, it can lead to severe dehydration or even pancreatitis. The longer a dog vomits due to this problem, the more important it is to see a veterinarian.
Toxin exposure can cause vomiting due to many reasons. Liver damage, kidney damage, direct irritation to the stomach, and irritation of the vomiting center in the brain
due to the presence of the toxin in the blood stream are all possible reasons. Ingestion of many toxins can be emergencies, Tylenol, ibuprofen, illicit (illegal) drugs, etc are all potentially life threatening if eaten by dogs in sufficient quantities. Some of the damage done by certain toxins to the liver or kidneys can also cause a dog to drink more. Unfortunately we often cannot diagnose ingestion of many toxins without knowing for sure that they were ingested. Certain test results can provide more information, but often we treat symptomatically to get dogs out of a "crisis."
Pancreatitis is inflammation in the pancreas. This is often caused by eating inappropriate or high fat foods. Supportive care is often successful at treating this problem, but severe complications do occur.
Because your dog is not eating
and can't keep water down, I would recommend that he be seen by a veterinarian as soon as possible. Many of the problems I have described above can be treated, but often a quick diagnosis is required to get dogs out of danger.
I hope this helps,