Forgive me for not looking through the forum first; I am trying to avoid any spoilers I may come across!
Anyway, I have the special edition of revelations, yet I cannot find the Vlad the Impaler prison anywhere (no secret location icon). However, I have unlocked the Ottoman Jester and the Crusader on mutliplayer, :s
I believe it's a templar icon you should be looking for in the map. Try to sychronize every single viewpoint first.
At one point in the story after meeting Sofia, a templar icon will appear in her library. There's like a book or note or something you have to interact with.
This will move the templar icon to the far west side of the island, where the entrance to the prison is located.