Hi Rewton,
For what it's worth, I spoke with Edible Landscaping in VA (7a) a few weeks back to buy a VdB for my parents for trial. However, EL said the PetiteNegri was coldhardier than the VdB/Negronne (EL considered VdB and Negronne to be the same thing). If memory serves, EL keeps the PN in-ground and pots their VdB. EL recommended covering in-ground PN's during heavy/prolonged freeze. I wound up going with the PN over the VdB, because I don't think my parents are up for the work involved with pot culture. Now if the little guy can just survive our rain/humidity.
Best of luck,
Rick