Well, I guess I really need to report this little glitch.
I've noticed that you can actually add from your inventory to an overstuffed vault.. adding both more bulk into the vault AND going over the allowed number of items/stacks. You can do this by splitting stuff out of your inventory and into any storage item in your vault, such as a backpack pouch, dragon scale pack, cargo disk. It will allow it, even if you are already at max in bulk and/or items in your vault.
Sorry to spoil it for others who may have noticed this, but I don't think that's intended to be possible.