My idea is 1 combined vault for all 5 characters of the account...
Vaultsizes:
When the player would have0 alt: 2k (main)=2k (total)
When the player would have 1 alt: 2k (main) +2k (1alt)=4k (total)
When the player would have2 alt: 2k (main) +4k (2 alts) =6k (total)
When the player would have3 alt: 2k (main) +6k (3 alts) =8k (total)
When the player would have4 alt: 2k (main) +8k (4 alts) =10k (total)
Each toon would have to upgrade its own part of the vault up to the full 3k.
It could/might create a better world for the fewplayers that do have alts but dont wanna be in a guild like myself.