Combine rice vinegar and wine in a saucepan on medium heat, then reduce until the liquid forms a light syrup. (If you are not experienced at making butter sauces, you can prevent the sauce from splitting by adding the optional cream.) Decrease the heat to low and whisk in butter, one piece at a time, until well incorporated. Add chopped fresh sage. Season to taste with salt. Set aside and keep warm. When reheating, be careful not to boil sauce as it may split.