by sheer happenstance, i figured out how to rob the castle vault, i'll explain how and what you get in the spoiler, though there are no actual spoilers that i'm aware of
SAVE FIRST as you will go to jail if you're spotted. wear the marcher armor you can buy from the sundries store (or find elsewhere) to look like a guard, at around the evening, you can follow Disa from her room into the vault, normally no one but her is allowed in, guard disguise or not, but if you barge in while Disa is passing the door guard, you can loot the place in a counter-clockwise sweep to avoid Disa and run right out, no questions asked
items: daughter of the evening (shield that can reflect gorgon sight) wyrmhunter cape 20,000 gold ring of reassurance (junk, boosts pawn defense after you revive them) decayed medusa head (no idea, you can equip it and use it to do... something, but it's single use, so just save it. rare material?)