How to merge item stacks?

Solution 1:

Items in this game are in random arrangement (in the order you acquire them perhaps) and cannot be sorted or stacked.

Don't bother organizing your inventories. Just store the items (vulneraries). Trick: give your characters weapons, then give them back their vulnerary.

It really doesn't look like vulneraries can restack. No matter what I try. It hasn't been a mechanic in other games either. (Edit: it was definitely a thing in Shadow Dragon and Awakening — reports scohe001)

Trick #2: Equip/unequip stuff on a character to get items in at least some semblance of order. (Armor first, then weapons, or best>good>okay>bad, or what ever organizational system you like.)

But, good news; they take up almost no space whatsoever. All stack into one item slot in the end, so in total there will be just 4 rusted weapon stacks. Hoard one of each weapon type as they can be useful later.

Don't bother to sell them, you only get ~1g. Waste of time for not much space freed at all.