Combine sugar, vinegar, ancho and orange peel in a medium saucepan. Cook over medium heat, stirring often, until sugar dissolves and mixture becomes a thick syrup, about 10 to 15 minutes. Remove from heat.
Preheat broiler on high.
Melt butter over medium heat in shallow saucepan or deep skillet (wide enough to hold all the halibut). Stir in the vanilla and then add the halibut, skin side up. Partially cover and cook for 8 to 10 minutes, turning once.
Remove halibut and place on baking sheet, skin-side down. Sprinkle with salt. Broil to 2 to 3 minutes or until light golden brown.
Place halibut on a serving tray and drizzle with sauce.