Welcome to the forum m8.
Your idea smells black lotus bazaar. I wouldn’t be surprised to be sold in the next update for 2600 Crom coins.
But no matter the size vaults are useful in pve too, no matter if the plurality of pve players do not use them, they are extremely useful. Especially in old purges, fixing 4 vaults in your back yard before purge was really handy to secure your whole fortune!