Hi
Careful with 3rd party vendor - some of them will use NDMP which i believe not supported. you need something that is purely file based.
• Tape backup or restore using dump and SMTape on the destination volume
• Tape based restore to the source volume
https://library.netapp.com/ecm/ecm_download_file/ECMLP2811525
How about just enabling AGGR dedup on the dest, and always maintain the two relationships?
Another idea i had is to maybe put another layer on top of the volume, like an ONTAP select (which will have the snapvault out). or a windows file server/cluster that will be backed up by traditional backup software you maybe already have...
Also - Just wondering also, why are you avoiding metrocluster over IP? cost?
Sorry if it's not really answering your question - just thinking out loud.