Trim ends from cucumbers; discard ends. Cut cucumbers into 1-inch-thick slices. Place in a large zip-top plastic freezer bag on a cutting board with cut sides of cucumbers facing up. Smash lightly with a meat mallet or heavy saucepan until cucumbers have broken into pieces. Transfer cucumbers to a bowl. Add vinegar, soy sauce, fish sauce, sesame oil, and crushed red pepper, tossing to coat. Cover and chill, stirring occasionally, 30 minutes.