I would assume they would add additional pages to the vault, as it works with your friends list and currently players list.
Another possibility is that they could split the vault into three different vaults. One for inventory items, one for weapons, and one for armor.
EDIT:
Many thanks for all the replies and opinions!
Can we make it to the trending page??
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Well lets see, we have 3 characters with 3 sections of 9 slots each. So thats 81 slots plus the 20 in the vault. We have 101 now, so another 86575309 slots should be appropriate