AOW:SM did not have a 'vault' but it did allow each hero to carry (not the same as equipping) items and allowed you to teleport items to any hero you wanted. Teleporting items cost an amount of mana based on how far away the target hero was. Loved this system.  

Or all three of the above methods. The "teleport artifact" could be a Magic Tree tech to allow you to do this.

The "packmule" could be a Military Tree Tech to allow this,

The Vault could be a Civ Tech or just the basic default however I like it as a tech. 

Why make a spell that teleports only one item? Why not make a spell that grants access to a common "stash" (like Diablo 3 or even like the extraplanar chest from Temple of Elemental Evil) with every use? Therefore each casting of the spell lets you deposit and/or withdraw as many items to and from the caster's party.

In any case, any sort of item sharing solution would be appreciated.

I keep bringing up that a teleport artifact item would be perfect. We have the mana, we have vast distances, we have different strengths of items. What is there left to ponder about?

Mana cost(teleport item) = Distance * Item Quality