You have to have all three dragon shards to open that door. I haven't tried vanish, so I can't comment on it. Sounds like it's protected, though
However, you can get to Erathsmedor from the other side, as some folks have found, by stacking up a ladder or vanishing in the right spot. I actually enjoy doing Dragonspire because of all the high-XP random monsters in there... it's a great place to level up!
Except for the occasional fuming fungus... I hate those almost as much as the salads