Preheat oven to 325° F. Score ham on a diagonal every 1-1 ½ inches with a sharp knife, then place ham cut side down on a rack set on top of a rimmed baking sheet.
Cook ham in preheated oven 15 minutes per pound, until the internal temperature is 130°.
Meanwhile, combine the sugar and mustard in a small bowl. Add just enough vinegar to create a paste.
Add glaze to the top of the ham, and allow to drip/drizzle down the sides - the glaze will melt in the oven and continue to drip down the sides, so no need to be perfect.
Return ham to the oven, and bake another 15 minutes. Remove ham from oven and allow to cool 10 minutes before carving.