In a liquid measuring cup, whisk together the vinegar, oil, sugar, mustard powder, garlic powder, salt, pepper, and cayenne, if using. Taste and adjust the seasoning, if needed.
In a large bowl, combine all of the vegetables.
Give the dressing another good whisk to combine everything, then pour it over the vegetables.
Stir well to combine, taking care to coat all of the vegetables.
Cover and store in the fridge overnight, giving it a stir a few times while it marinates. Stir well before serving.
Notes
Easy storage and packing--Keep your marinated raw vegetable salad in a resealable bag.
Dressing ratio--If you prefer a less tangy dressing, reduce the vinegar to ⅓ cup and increase the oil to ⅔ cup.
Let it warm up--If your dressing clumps up after chilling, let it sit at room temperature for 10 to 15 minutes, then give it a good stir.
Storage--The salad will keep well for up to 5 days, if stored in the refrigerator.
Dietary--This recipe is suitable for gluten-free and dairy-free diets.