Combine vinegar, water, sugar, and salt in a small saucepan and bring to a boil, stirring frequently. When the sugar and salt have dissolved, set the pan aside to cool.
Place red pepper flakes, garlic, cilantro, and avocado slices in a mason jar. Pour the cooled pickling mixture into the jar and seal tightly with a lid.
Refrigerate for at least 3 hours to allow flavors to develop before serving.