its been stated that Vault space is a technical limitation
Namely the weapon comparison stat card data doesn't all fit into ram simultaneously when inspecting a proposed enlarged Vault
Here's a hilariously clever solution Bungie
Since the Cursor has a set max movement speed and is not truly random access, we can take advantage of the time delay to load different sections of the Vault into ram cache
Just reorganize the Vault weapon data into a binary space partition tree, then collision check cursor position against it to decide what to load
Easily cut the limit by half even in worst case
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It's simple. Just have each category be a vault. You could have 94 spots for primaries, 94 for secondaries, etc. (96 is total spots in the vault). And just load them when you load the category. So when you initially open the vault you'll see all armor categories ion the left, all weapons, in the middle, and consumables on the right (maybe Shaders, Consumables, Emblems as categories)