Mix the vegetable broth, mustard, herbs (the more the better - I mostly use mixed frozen herbs from Provence here), olive oil and sugar until it becomes a tough, sticky green mass. Add the yogurt and mix again. Now add the vinegar and mix again. Finally, add water depending on the desired consistency of the dressing.
Then season to taste and soften or sharpen with vinegar and water to taste. If you want, you can also add garlic or garlic oil - this gives the dressing a special kick.