Heat vegetable oil in a small skillet over medium heat. Add garlic; cook and stir until softened and lightly browned, 3 to 5 minutes. Remove from the heat and let cool.

Whisk vinegar and sugar together in a salad bowl until sugar has dissolved. Whisk in chili-garlic sauce, fish sauce, coriander, red pepper flakes, and cooled garlic until thoroughly combined. Stir in cucumbers, peanuts, and green onions until coated with dressing. Refrigerate for at least 1 hour or overnight before serving.