True that vomiting removes all of your stomach contents?

Not necessarily: Could happen but not necessarily. But if you are repeatedly vomiting, more testing is needed.

Sometimes: Sometimes the reason for the vomiting is retained food (e.g. In diabetics) because of an inability to empty properly - this is known as diabetic gastroparesis. Other times, e.g. Food poisoning or infectious gastroenteritis, the body is working hard to empty the stomach completely.