"Where is Vietnam?"
Translation:Việt Nam ở đâu?
Duolingo tells that "Đâu là nước Việt Nam?" was a possible answer. What does nước / water has to do with it?
ở means "to be located at", so you need it here, because there is no other verb in the sentence. If it's the only verb, and the sentence is about someone or something being somewhere, you basically need ở.
When something else is happening, if another verb is used in the sentence, like eating, sleeping, etc., it is also usually used. But sometimes you can use prepositions indicating a place (like trong, trên, etc.).