If a vault becomes over-collateralised after a partial liquidation, it is still subject to further liquidation as the auction isnt closed. The vault owner has to call cancel() himself, or trust other altruistic actors to perform this action on his behalf. Liquidators will unlikely do it because they are economically incentivised not to do so.
One can however argue that this is mitigated by the fact that protocol (governance) sets the vault proportion that can be auctioned. Regardless of whether the fact that the vault is over-collateralised after partial liquidations, the liquidators arguably are given the right to carry out further liquidations up to the proportion set.
Nevertheless, a reason for a revised liquidations witch contract is that More often than not, liquidated users have lost all their collateral as we have failed to make liquidations competitive.. Hence, it might make sense to ensure that users need not lose more collateral than necessary.
Consider checking if the vault is over-collateralized (maybe in _updateAccounting()) and close the auction if it is. This however adds complexity to the liquidation logic, as you have to update the cauldron first cauldron.slurp() before checking and updating the collateralization status. It will also break the CEI pattern, which might be unfavourable.
