Okay, there may seems to be some conceptual misunderstandings from my part!
If a region is occupied by data and the data is deleted on higher level (I deleted all the VM from the vSphere Datastore before testing), new data is then perceived as modification to the regions created from the VV. Therefore "new" data could be written to NL as long as the region on NL disks exists.
I did not wait long enough after I zero filled the Datastore and therefore regions in the NL disk still existed.
This is a lesson learned for me
Please correct me if I'm wrong