Its just plain white (not metallic) so a competent painter should be able to paint that front section of the roof for you and blend it into the existing paint - especially so as its on the roof of a van that no-one will ever see.
Otherwise a cheap temporary fix would be to buy a strip of black (or white) vinyl to cover that front 20cm or so of the roof to cover up that paint imperfection.
Its likely to be caused by a bird poo/tree sap that sat on the paint too long, or possibly poor preparation at the VW factory before the car was painted. I have read on here of other VW van owners having issues with paint cracking/peeling on the roofs.
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