Heat vegetable oil in a small pan over medium heat; cook and stir garlic until softened and lightly browned, about 5 minutes. Set garlic aside to cool.
Whisk vinegar and sugar in a salad bowl until sugar has dissolved; add chilli-garlic sauce, fish sauce, coriander, cooked garlic and chilli flakes until thoroughly combined. Stir in cucumbers, peanuts and spring onions to coat with dressing. Chill at least 1 hour to overnight.