The good news! I managed to get the majority of the cave done!
The bad news... I now lack any idea of how to fit together the entry point of the Vault into the interior of the cave! >.<
Sometimes, I really wish I thought these things through better... either I'm going to have to redesign the cave using the insanely limited, intended-for-vault-use pieces... or I'm going to need to do some serious finagling to get this to work.
Well, if you mean the huge vault door entrance... that in fact has a limited cave piece and it doesn't link up to the regular cave pieces to my knowledge.
If you must use that huge circular vault door, the easy way would be to do something like this. You could make a regular sort of door out of your cave area into a different area - - that area could either be be a small room-ish place, or, it could be a seperate interior cell entirely. THAT area would link to the actual cavey-vault-entery pieces (and the door which fits there).
The harder way is to use the vault entrance door even though it does not fit, and use pieces (such as boulders and tilted landscape maybe, and similar) to completely disguise the fact that the pieces do not fit. But if you're new to fitting cave pieces together in the first place, trying to do that might piss you off.