Prepare a grill for high indirect heat (for a gas grill, leave one or two burners off; for a charcoal grill, bank coals on one side); oil grate. Bring garlic, lemongrass, sugar, vinegar, red pepper flakes, cumin, black pepper, and ½ cup water to a boil in a small saucepan, stirring occasionally, and cook until reduced to ½ cup, about 8 minutes. Let cool and stir in fish sauce. Transfer half of sauce to a small bowl and set aside for serving.