In a plastic container that has a lid, add the vinegar, vegetable and olive oils, Dijon mustard, garlic, onion, salt, black pepper and sugar. Cover the container with a lid and shake for 10 seconds until the vinaigrette is well emulsified. Alternatively, you can also whisk with a fork in a bowl.
Place the lettuce, carrots, cucumber and tomato in a large salad bowl. Pour a generous amount of the vinaigrette on top and toss. The salad should be coated with the vinaigrette but not swimming in it.
Top with avocado slices, drizzle a bit more vinaigrette on top of and serve.