1. Can we create SnapVault snapshot copies from replicated vols or all snapshots have to be snapmirrored from source vols?
A SnapVault backup is a collection of Snapshot copies on a FlexVol volume that you can restore data from if the primary data is not usable. Snapshot copies are created based on a Snapshot policy. The SnapVault backup backs up Snapshot copies based on its schedule and SnapVault policy rules.
2. in case the data corrupted in destination vol, are we still able to restore data from snapvault copies?
The destination volume is usually read-only until the snapvault or replication backup is broken. But to answer your question, as long as the snapshot is healthy you should be able to use to restore either from secondary or primary.
3. if we keep snapvault for years and data changes big and often, will the destination vol need a lot of space to keep snapshot copies?
Technically yes, the snapshot policy created will determine how many to keep.
4. I assume normally snapvault copies will only be kept on destination, if big size vol restore will be needed, will it take too much time to recover?
Restoring from a snapshot is usually an instant process.
5. we plan to make tape copy on source site, is that a good idea compare to destination since it is away from production site?
Having an extra copy of the data is always recommended. But, that would be something you would decide, there is no harm with extra backup.
I have provided below some documentation about Snapvault (Not sure which Data Ontap You running):