Thanks for the additional information. I really appreciate it.
So the thing I worry about is the possibility of an intestinal obstruction.
You have every right to be concerned. Your dog is showing some symptoms of an intestinal obstruction. The attempting to vomit with no success and firm stomach in the days after ingesting a piece of rawhide are HUGE warning signs that your dog needs to be seen by a vet immediately.
An intestinal obstruction (likely caused by the rawhide) is what happens when the dog ingests something that is too big or too bulky to pass through the digestive tract the normal way. It gets stuck on one of the turns in the intestines and stops the flow of everything, causing your dog to get sicker and sicker until the intestine ruptures (if left untreated).
This is something that is absolutely, positively worth the drive to the emergency vet (unless your vet is still open, in which case you could call the). Since your dog is already showing signs of obstruction I would strongly urge you to NOT wait until morning to have him seen...intestinal obstructions can turn fatal very quickly, so time is of the essence here.
At the emergency vet, they'll take an x-ray to see where the obstruction is and if it's still high up in the digestive tract, they'll have to remove it surgically.
You can read more about obstructions below:
I hope everything works out.