This could be a number of issues. I would like to see a snap list on both the source and destination. I would also use the command snapvault destinations on both the source and destination and snapvault status on both systems.
When we snapvault from a SM destination we create a sv_daily snapshot schedule on the source and key the snapvault to that snapshot not the snapmirror snapshot. I am not sure how the NetApp software works because we don't use it.