I went to that cave already. The first time went there, their was a flashback about Vincent and this girl (I forget what her name was), and I didn't get any key.
I went back there a little later and then i got vincents final limit break. I still havent got any key. I want to know if I messed up big time, or what elce do I have to do? And yes, I went back to bugenhagen and it still says I have to look for the key.