To keep your salad crisp and vibrant, add the vinegar to the greens before you add the oil. The reason it's beneficial has to do with the lettuce itself.
Lettuce leaves have a tough, waxy exterior that keeps liquids from penetrating the leaves' surface and making them wilt. Oil can sneak past this layer and break down lettuce.
Adding vinegar to your salad first creates a barrier between the lettuce and oil, ensuring the lettuce retains its crisp, fresh texture for a longer period.
This rich, heavy ingredient will also coat all your salad's ingredients and prevent the vinegar from sticking to the salad veggies, leaving it to pool at the bottom of your bowl.