Actually, you might only be able to limit unit design items to Vaults of the same number and City Type.
SECTION 1
However, to do this I think you would need to have 9 different custom resources (Local to the city they are in):
- the requirement to have a certain level would mean you would need to have the Vault 1 Items require a resource produced by Level 2/3/4/5 City Hubs of each City Type (one Resource for Town Vault 1s, one Resource for Fortress Vault 1s, one Resource for Conclave Vault 1s), Vault 2 Items would require a resource produced by Level 3/4/5 City Hubs of each City Type (another 3 custom resources, one for Vault 2s at each City Type), and Vault 3 Items would require a resource produced by Level 4/5 City Hubs of each City Type (another 3 custom resources, one for Vault 3s of each City Type). 9 New Custom Local Resources.
SECTION 2
So to add an item to a unit design:
- the item will have to be unlocked (which it will if it is one of the items that is randomly selected to be produced by a Vault - this would not be unlocked until the Vault is finished building, an important point).
- the item will have to be available for unit design (I'm not sure if the +hitpoint items would be able to used for unit design unless they were modified to be appropriate for trained units, the +Physical Attack items would only be in if they were particularly expensive)
- the local resource produced by the City Hub (which means you have the City Hub requirement)
SECTION 3
The actual Town/Fortress/Conclave Vault 1/2/3 building itself would require all the following three conditions achieved:
- the Tech (as of now, Glyph Stones for Vault 1s, Armor for Vault 2s and Arcane Armor for Vault 3s)
- correct spell for that particular City Type and Vault Number cast on that city
- City Hub of the minimum level or higher for that City Type and Vault Number.
Each City Hub as discussed in the first section would have a certain number of a unique accumulating (eg. metal, crystal, horses, wargs, beer) or non-accumulating (eg. Shard Power) resource (a large number if you want to allow virtually unlimited training/retraining of troops and the resource does not accumulate).
The City Hub of a certain level and a certain type would produce the local resource which would only be required by Vault Items of a particular City Type and Vault Number.
Simple idea, but very complex execution! Would you like some help to get the unit design thing set up for your Vaults?