Combine the rice vinegar, olive oil, sugar, lemon juice, and salt in a small bowl. Toss together the cucumber, radish, shallot, and cilantro in a medium bowl.
Pour the dressing over the mixed vegetables and toss until well coated. Add the sesame seeds and mix one more time. Place in the refrigerator and let it set for at least 1 hour.
Serve this salad the same day it is made. The next day the veggies will start to break down due to the vinegar.