Sorry, I missed your reply. The problem is that you in the “Build query” part queries the child list for items where the field _vLookupParentID is equal to the current items ID, but you are not transferring the ID to the child item in the “Prefill values in child”.
If you want to use the parent items ID to link to the child items you must change this line
From _vLookupID to _vLookupParentID
to
From ID to _vLookupParentID
Alternatively you can change the Build query “Search value” from [currentItem:ID] to [currentItem:_vLookupID]
Alexander