I guess this is understood: Only one SnapVault relationship is supported in a cascade chain, however multiple mirror relationships are supported.
I think the user is doing : A --> Vault --B --Intends to do Mirror --> C (Should be ok). As long as he is not providing qtree as replication dataset which could be the reason for that error.
However, if user will be able to re-establish vault between A to C is something I am not sure, and that's why I think Aborzenkov's Technote looks appropriate to me, as it says - Scenario 2 : If moving individual Qtree SnapMirror, SnapVault, or OSSV destination qtrees from one destination volume to another. That is what user's ultimate goal I thought : As the user is running out of storage on System B.
@Hitesh123: Could you clarify that?