A vault is a contract each user generates for their wallet address, which they are the owner of. This contract can hold NFTs and fNFTs in a custodial state for both sales and auctions. An individual's vault its owner to sell their NFTs from the vault if they wish as well as withdraw any NFTs/tokens received from airdrops at any time as well as making things simpler to grab information like the number of unique holders & true owners of NFTs.