If there is any oil in it – then it is fattening. If you like oil in it – make sure you choose the dressing with Olive Oil since it is considered to be a “good fat”. I know there is a lot of fat free vinaigrettes – Wish Bone makes a good fat free red wine vinaigrette dressing. If you are just using the plain vinegar, then you are choosing the most healthiest way to go.
You can easily make it low in fat yourself too. I usually make it myself because I can control how much oil I put in. I don't like very much oil in my dressing. I put in very little olive oil and use flavorful vinegar like red wine, balsamic, or rice vinegar with a clove of garlic, lemon juice, and spices. I'm sure you could put in any flavor you want.