Combine the vinegar, sugar, crushed red pepper, salt and garlic in a medium saucepan over medium-high heat and bring to a boil. Reduce the heat and simmer until the sauce thickens and can coat the back of a spoon, 12 to 15 minutes. Remove from the heat, then add the butter and whisk to combine. Adjust the seasoning if necessary. Use on wings cooked to your preference.