Note that vomiting is not a common symptom of dehydration, although it can make the condition rapidly worse. Something else might have been going on with your daughter in addition to not getting enough fluid. It's worth a trip to her doctor's office. Untreated dehydration can lead to shock and can even be fatal. Fortunately, this is a problem that you can prevent.
Water is fine, but given a choice, children often prefer sports drinks.
A sports drink is best to restore electrolyte balance, but avoid those with artificial colors and other synthetic ingredients. If you don't have any on hand, mix a half-teaspoon of salt and eight teaspoons of sugar with a liter of water. This will do until you can get a sports drink to replace lost fluids, sugar, and salts. Water, juices, carbonated beverages, broths, or gelatin desserts will replace lost fluids but won't restore electrolyte balance. Dr. Andrew Weil Dehydration can sneak up on adults as well as children in hot weather.