So you are right.
The technologies are designed for slightly different requirements.
SnapMirror is a DR solution and is used to ensure you have an exact mirror of the data in production on a secondary filer, so all data, snapshots, snapshot retentions are identical at both ends.
SnapVault however is a backup technology so you can have different retention rules etc on your backup site to those housed in production.
So very much down to your requirement, you can however do both you can vault and mirror or mirror and vault, depending on what your needs are.