Thai Cucumber Salad

3/4 cup distilled white vinegar
1/3 cup granulated sugar
1/2 to 1 teaspoon red pepper flakes
1/4 teaspoon kosher salt or margarita salt
1/4 cup finely chopped red onion
4 medium cucumbers, peeled and cut into 1-inch slices
1/4 cup salted peanuts

Mix all ingredients except peanuts. Add peanuts just before serving.