You can convert QSM to SnapVault with assistance of technical support, but no such option for VSM. BTW, why don't you consider to take and leave snapshot on source volume? If you have enough space for keeping snapshot on source, this is easiest way. VSM covers all the snapshots on source. When you need to restore from older snapshot, restoring by snaprestore from source snapshot is first option. If source volume is corrupted, resync from dest then use snaprestore.