"...a "Vatican source" told National Catholic Register March 22 that survivors who write to the Vatican should receive a response from their local bishops, not Vatican offices."
In an previous article in NCR with Cd. Mueller's response to Marie Collins, Cd. Mueller says in regards to having the Vatican congregation [CDF] respond to the letters,  "... [it] would not respect the legitimate principle of diocesan autonomy and subsidiarity."  
So the 'Vatican source' is no secret. Cd. Muller in an inteview and the 'Vatican source' said essentially the same thing.