Both are methods of replication data from source to destination.
SnapVault, Qtree level replication
SnapMirror, volume or Qtree level replication
SnapMirror, make the destination volume read/write with one command (snapmirror break)
SnapVault, destination volume read only
SnapMirror, destination volume is the exact mirror or source volume
SnapVault, several versions of the source data can be kept at the destination
For volume SnapMirror the source and the destination volumes must be the same size
For SnapVault the source volume can be larger than the destination volume
So both are technologies used for different scenarios. They can also be used together in one environment