50% in most cases
an overwhelming majority of von Willebrand
Disease (VWD) is dominant, meaning that if you carry one gene, you have VWD.
Supposing that your girl friend has one VWD on one chromosone, and the other is normal, the chance of her passing that VWD gene to your offspring would be 50%. Most patients with VWD have type 1 disease which is fully compatible with normal life span.