Salad dressings need to be targeted for dilution or they won't dilute as much as you want.

If it's homemade, just use a little more salad oil

If it's bought outside, just add water and mix well. You can also use milk or water to dilute the dressing, add a little bit at a time, mix well after adding and then continue to put a little bit more, then mix well again before adding.

Alternatively, you can try adding some low-fat yogurt or vegetable puree to the salad dressing to dilute it, which will not affect the flavor of the dressing and reduce the amount of fat consumed.