Sounds like your dog is pretty sick. Is he drinking fluids, water, Gatorade, broth? It is I portent he stay hydrated. Dud this Vet do any blood work, test stool samples or take x-Rays? If I were me, I would be seeking the advice of another Vet and soon! Good luck and please come back to update.
He could have some bleeding high in the gut, and that could be the cause of the dark coloured stools. 5 days vomiting regularly is not good. Dehydration is a definite risk.
Can you buy unflavoured Pedialyte where you live? If not, try water in which a little honey or maple syrup has been dissolved. Then don't let him drink lots at a time, but very small amounts very frequently.
If he won't drink, use a syringe (without a needle!) or plastic turkey baster (not glass) and syringe small amounts slowly into the corner of his mouth every half hour. Hopefully he will keep it down. If not that is fast becoming an emergency situation.
I agree with Margot. He needs tests. I am surprised your vet hasn't done them already! I would consider taking your dog a.s.a.p. to a new vet.
I second what Margot and Ginger said. I would also add that If it were mine, I would use the same syringe technique, slowly in the corner of the mouth, with a mix of 1/2 water, 1/2 milk with some acidophilus (pro-biotics) mixed in. You can also get pro biotics in liquid form at health food stores if you prefer that. Not cheap, but neither is the vet or drugs. He may not keep that down either, but to me it would be worth the try. It certainly can't hurt.