SnapMirror is disaster recovery technology, designed for failover from primary storage to secondary storage at a geographically remote site. SnapVault is archiving technology designed for disk-to-disk Snapshot copy replication for standards compliance and other governance-related purposes.
There is plenty of documentation about the diferences on the NetApp knowledgebase and/or Internet.
More latest info (Policy types section):
https://www.netapp.com/pdf.html?item=/media/17229-tr4015pdf.pdf