In the "old days," there was a technological difference between the replication engines used by SnapMirror using an async-mirror policy and SnapMirror using a vault policy, but that has changed such that both "SnapMirror" and "SnapVault" use the same replication engine. The only difference is in what each of those policies allows. An async-mirror policy does not allow user-defined SnapMirror rules that tell SnapMirror which source Snapshot copies to replicate along with the "active" file system. A vault policy only backups up the source Snapshot copies, but does not continually update the source's "active" file system. A mirror-vault policy allows both.
For all intents and purposes, "SnapVault" is simply "snapmirror with a vault policy".
For more details, you can download TR-4015