In a separate, medium-sized bowl, combine remaining ingredients. Stir until sugar dissolves.
Pour vinegar mixture over cucumbers and onions. Stir until the cucumbers are evenly coated in the dressing.*
Refrigerate at least 20 minutes. Before serving, drain liquid and place in a serving bowl to serve.
*You can also combine the cucumbers, onions, and dressing in a resealable plastic bag to marinade. Drain liquid from the bag, and pour the salad into a bowl to serve.*See notes above for instructions on saving in refrigerator.