"Where are you?"
Translation:Vos ubi estis?
It's a synthetic language, not an analytic one.
Synthethic language: word ending changing according to the grammar role in the sentence, less prepositions, and fewer words.
Analytic language: more words, as the word ending doesn't change, and it's the word order that convey the meaning, not the ending.
"Vos ubi estis?" is certainly right when you want to insist on the "you", like it's the case in Spanish and Italian.
To my understanding, adverbs (like ubi) should usually precede verbs (like estis) in Latin. You should try to follow the various word order conventions in Latin (like adverb before verb, noun before adjective, preposition before object, etc.) in everyday sentences like this—the reason "not strictly" was used (I would guess) is because the conventions are often broken in certain forms of writing (like poetry). Latin can technically be understood in pretty much any word order, unlike English, but normal sentences just sound a bit odd if conventions aren't followed.
Our your sending an epistula out to your milites on campaign. You tell the tabellerius to travel the route until you find them. In the letter you right. Quo estis. They then reply what their urbe proxima is. Letting the patrones back in Roma know when to expect their adveniunt.