I understand your plight when it comes to homemade low fat salad dressing. I also use a low fat version, but not as little as a few tablespoons of oil. The problem I find going that low on oil is the need to sweeten the vinegar for it to be palatable. All that straight vinegar is hard on the throat, which is why I'm assuming you add the water. I may be repeating some of the advise already given, but it will be in a different context.
Instead of water, may I suggest adding 1 to 3 teaspoons of honey. Not only will it sweeten and smooth out the vinegar taste, it will also help to thicken the dressing. An alternative option is a Balsamic reduction. Now I know what you thinking, it will only be stronger. However, reducing "Balsamic", will actually sweeten and thicken it. Using the higher amount of honey, you can try using other vinegars, but make sure it is a really good brand. Treat it like wine. The better the brand, the smoother the taste. Try white wine vinegar with the honey. Also try replacing 25% to 50% of the vinegar with tangerine juice and a tsp of it's rind.
Lastly, you generally use less dressing when it's low fat which can make it harder to distribute. I always put the dressing in the bottom of the bowl first, then add cut tomatoes in next and all other veggies on top. The juice from the tomatoes adds to the dressing. Toss well by going down the sides of the bowl, then pull to the center and pick up. Believe it or not, it is a technique that works. I hope this helps.