Only the vault with the lowest collateral ratio can be redeemed against for any given vault type. However, there may be multiple collateral types (e.g. SOL and BTC) each with separate types of vaults; if both exist, the vault with the lowest ratio per collateral type can be redeemed against. Redeeming against a vault with a higher collateral ratio (i.e. a vault in better standing) leads to a higher fee. Therefore, this fee incentivizes users to redeem against the most under-collateralized vault across all vault types and, in turn, for vault users to maintain healthy collateral ratios.