Just read this~ "Depending on what level you are this step will take along time. If you are 70+ it could take minutes; if not it will take hours. In order to get a legendary dragon to appear you must be level 70 or higher and it will just be a random dragon assault so it just comes down to exploring while waiting for the legendary dragon to appear." So in order to get the Achievement/Trophy (Whenever PS3's comes out) you need to be level 70+..
I think it's a god idea.. Do yu think you could bceome a werewolf in Dawnguard, get all 11 perks, cure your lycanthropy at you know where, then become a vampire naturally in skyrim and get all vampire perks?