Hi Muhammad.
Sorry to report, but I never found a solution to this one.
We managed to "work around" a few of the relationships by totally destroying the destination and re-creating the SnapVault / SnapMirror relationship.
I know it's not an ideal situation, but it's the only one that has worked for me.
We originally had the issue with DFM 3.8. But the customer has since upgraded to DFM 4.0 and still experiences the issue intermittently. Although, after a day or three the relationship eventually shows up in DFM external relationships.
Mike.