When an item is not accessed for a day or more value cache will continue to collect its history data and will remove it from cache only when it rans out of free space.
However if the space is needed to store more data for such item, then this item is not removed from cache. In theory this might cause value cache to switch to low memory mode even if there are some space that might be freed without problems.
This could be easily fixed by removing the item instead of adding new values if it has not been accessed during last 24 hours. This would also make unused item cleanup redundant in vc_release_space() function.