Maybe I've somehow overlooked this...several times. I can't seem to find the Search DC for locating the door to the Treasure Vault. I've read through all of the room descriptions, as well as the introduction to the caves, and I can't find mention of it anywhere.
Hmm, I can't seem to find it either...
James might tell you differently--if he doesn't answer back, I'd set the DC at 20--this way the party finds it if they search carefully but probably misses it otherwise. It's a good standard DC for secret doors in low-level adventures.
Thanks for calling attention to this, as I'll probably be running this part of the adventure Friday.