The day before, cut up the vegetables and chill.
About half an hour before serving, take a break from curry-making to mix the cucumbers, shallots, and chiles in a deep bowl. In a separate bowl, whisk together vinegar, sugar, and salt until sugar has dissolved.
Pour liquid over cucumbers and let pickle 15 minutes to 1 hour (they should be fresh and crunchy).
*Thai chiles are very hot; seed them for a milder pickle.