So, definately being SnapVault volume, I therefore think the warning you might be seeing could be related to enabling compression, from the Data Protection Using SnapMirror and SnapVault Technology Guide...
If the secondary volume has additional compression enabled, storage efficiency is not preserved
Storage efficiency on all data transfers in SnapVault relationships is not preserved when the secondary volume has additional compression enabled. Because of the loss of storage efficiency, a warning message is displayed when you enable compression on a SnapVault secondary volume. After you enable compression on the secondary volume, data transfers are no longer storage efficient. You can reenable storage efficiency on data transfers after you disable compression on the secondary volume.
Therefore you'd just want to match the primary storage efficiencies to maintain the savings ensuring all data is transferred efficienctly; you should not see any error messages/warning. For SnapVault destination volumes you can and should (unless there are specific reasons) enable the storage efficiencies.
You can also safely ignore the messages about the snapshot overflow since this is by design, once acknowledged the red will go away. It is easier to manage with a 0% snap reserve, since you only need to alert against the volume capacity.
Let me know if you have any other query.
Thanks,
Grant.