Combine vinegar, mayonnaise, dill, sugar, salt and mustard in a small bowl and whisk to combine.
Peel cucumbers and split in half lengthwise. Using a spoon, scoop out the seeds and discard. Slice the cucumbers, about ¼-inch thick. In a medium bowl, toss the cucumbers and sliced onions with the dressing. Refrigerate for 15 to 20 minutes before serving. Serve chilled.