Combine sugar, vinegar, wine, water, spices, vanilla and zest in a pan over low heat, stirring until sugar dissolves. Bring to the boil, then simmer until reduced by half. Add plums; cover with a lid and cook over very low heat for about 15 minutes. Using a slotted spoon transfer plums to a bowl. Return pan to medium heat and simmer for 5 or so minutes until the mixture is syrupy. Add syrup to plums, cover and refrigerate until chilled and ready to serve