@maitre in Windows parlance, DFS areas appears to NetBackup as if it were plain NTFS structures - but DFSR areas appear to NetBackups as subsets underneath Shadow_Copy_Components.
So, backup DFS areas as if they were normal folders, and NetBackup will engage VSS to do the right thing.
And, backup DFSR areas as if they were under Shadow_Copy_Components, and NetBackup will engage VSS to do the right thing.
Do you see how in both cases... that NetBackup engages VSS to do the right thing. i.e. you yourself should not need to mess around with VSS or with Shadow Copies.
However, if you are trying to do something special with VSS and/or Shadow Copies outside of the control of NetBackup, then you may need to ask a question elsewhere, perhaps on a Microsoft forum.