Preheat the oven to 200°C (400°F). Put the bones in a roasting tin and roast, turning once, until brown. Transfer to a deep heavy-based saucepan, add the chicken feet and 300 ml (10½ fl oz) water. Cover the pan and cook over low heat for 6 hours. Strain, then return to the saucepan and reduce for a further 2 hours until viscous. Stir in the vinegar. This is now the duck glaze.