The Vatican has several reasons to conveniently misplace or lose valuable information about humanities past. This diary would be so interesting to read and so valuable a commodity for historians around the world. Being the history buff I am I would love to dig into Trajan's personal doctors diary.
Having said that the Vatican has a unbelievable library that seems to just go on and on forever. The Church has held power on this planet for a very long time. Even if I were given a chance to go into this library and had the knowledge to read the complex languages of the world, I would most likely would not find what I was looking for. There is just so many books, scrolls, diaries, and ancient manuscripts that it would be hard to find any proof of the things we wonder about today.
I truly believe however that things like the Fatima Prophesies were not fully divulged to the public. They are still sitting in some secret vault in the Vatican Library. Things like the Knights Templar and the holy grail are sitting in the Vatican library still today. Even perhaps the secrets of the old alchemist and the philosophers stone. Because this knowledge is too powerful in the opinion of the powers to be we may never truly know the secrets of Dacian Draco and the Kingdom of Dacia.