In oil-vinegar dressings, you should have a note: First vinegar in the bowl, then the oil slowly but steadily add. And it vigorously stir.
Only then mix vinegar and oil properly (and you get not only a leaf salad with oil over).
To a good salad dressing to make you do not need expensive „Shaker“. There are a small bowl, fork (for agitation) and a spoon (to measure).