"Really? You wrote to him?"
Translation:Vraiment ? Vous lui avez écrit ?
Not really, because these 2 questions are actually one: "You really wrote to him?" = "Tu lui as vraiment écrit ?" but asked in a way that is hardly a question. It is more an expression of disbelief or surprise.
Also, I would think that the imparfait is not right here and that that you should keep the same register of speech from one language to the other.