Dressing essentials. Salad dressing is an emulsion: a mixture of two unblendable liquids, in which one liquid (in this case, the vinegar) forms tiny droplets within the other.When the vinegar is suspended in the oil, it forms a thick, glossy mixture.l Vinaigrette-type dressings are unstable – the oil and vinegar will separate again if they are left to stand around for too long. A good dressing should complement the salad, and not overpower it.hen the vinegar is suspended in the oil, it forms a thick, glossy mixture. l Vinaigrette-type dressings are unstable – the oil and vinegar will separate again if they are left to stand around for too long. A good dressing should complement the salad, and not overpower it.