In a 2-cup measuring cup or a medium bowl, combine the vinegar, sugar, fish sauce, red pepper flakes (if using), lemongrass, garlic, cilantro, soy sauce, and sesame oil. Whisk all together. Taste and add salt and pepper according to taste. (I didn’t add any salt to mine. The fish sauce and soy sauce was enough for me.) Also, taste for any extra additions of the other ingredients, and adjust as desired. Set aside while you prepare the vegetables.
Slice the cucumbers ½” thick and place them in a medium bowl.
Add the chiles (if using), radishes, and green onions.
Give the dressing another stir and pour over the vegetables.
Toss everything together, coating well.
If serving immediately, top with the crushed peanuts.
If not serving immediately, keep the vegetables and dressing separate; cover each and refrigerate until ready to serve. Then toss together and top with the crushed peanuts just before serving.
Refrigerate any leftovers for up to 2 days.