Few things about the vFiler DR mirror relationship in Protection Manager(PM):
1. PM does not support the vFiler DR mirror relationship management.
2. There is vFiler DR plugin that can help you View, Create, Edit, Start, Stop, Protect, Activate, Resync and Delete VFilers.
3. If the vFiler SnapMirror relationships are created using physical controller interface than PM can discover the relationships as external relationships and you can still monitor the lag status. This lag status falls under External relationship lag status & global threshold(default 1.5 days but configurable) will be used for alerting purpose. However such relationship cannot be added to the datasets(existing and new).
4. If the vFiler SnapMirror relationships are created using vFiler interface than PM cannot discover the relationships and hence no way to manage it.
Therefor if I understand situation here correctly than it seems that you won't be able to manager the vFiler SM relationship in a dataset that you already have(for Backup).