From Real Vegetarian Thai by Nancie McDermott. In the note it says that if you are using a nice, unwaxed cucumber to leave some of the skin on. (I never use the cilantro.) This goes well with Thai Shrimp With Coconut-Almond Rice, Recipe #287896.
In a small saucepan, combine vinegar, water, sugar, and salt and boil gently over medium heat. Stir occasionally to dissolve the sugar and salt. After about 2 minutes, when the syrup is clear and slightly thickened, remove from heat and cool to room temperature.
Peel the cucumber and slice lengthwise in quarters. (You can remove the seeds if you want to.) Slice these strips into 1/4 inch thick triangles.
In a bowl, combine the cucumbers, cooled vinegar mixture, onion, and cilantro and mix well.
Use a slotted spoon to serve into individual dishes. Sprinkle servings with the chopped peanuts and cilantro leaves.