You need a chemical factory to make those dressings ....... how about just oil and vinegar? 3 parts oil to one part vinegar is the classic ratio. I use a good quality olive oil, then you could experiment with different vinegars - balsamic, white wine, cider. For added flavouring you could use mustard or tarragon or garlic or chilli or ginger etc, etc. If you don't like vinegar, you could use lemon juice instead.
The trick is to only use a small quantity of dressing if you are counting calories, this is far healthier than buying a "fat free" product which is composed largely of gums and thickeners and assorted "E" numbers.