Using a rolling pin, smash whole cucumbers lightly, just enough to break open. Tear or cut the cucumbers into 1 1/2-inch pieces and season with the salt. Transfer to a sieve and let drain 10 minutes. In a large bowl, whisk together the vinegar, soy sauce, sugar, chili oil, and sesame oil; stir in the grapes and scallions. When cucumbers are done draining, add them to the bowl with the grape mixture and stir to combine. Sprinkle with sesame seeds and serve.