The house vault is extra storage space on your character that you access from your house. It was never intended to be shared storage space for multiple characters.
What other things do you want trustee's to be able to do that they can't already? Are you sure they can't interact with items in the house? They should be able to use most items except for things like mirrors. Those are intended to be rewards for the owner, and not something you can buy and share with a bunch of other people.
If you can, give me some specific examples of things that your Trustees can't do that you think would be helpful.
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(Rothgar) Re:Why no co-habitation full access option?
