The Guild vaults are unlocked when upgrading your guild hall.
Basic level: no vault
10 000F: 1 vault
100 000F: 2 vaults
250 000F:3 vaults
Guild vaults can be used to put items at disposal of other guild members. They cannot hold souldbound items.
This idea is quite simple. Even if it could be used to get private vaults some way, I think they are hard enough to get and splitted between enough players so that it is not a big storage advantage, but an easy way to get items flow between guild members.