An optional ingredient you might consider having on hand for dressings is xanthan gum. In *very* small amounts, it's a great emulsifier, which helps keep the oil and vinegar parts of your dressing in suspension together, i.e., emulsified.
For this quantity of dressing, slurry about 1/8 teaspoon xanthan gum with half a teaspoon of whatever oil you're using, then pour into the dressing and whisk or blend for 2 to 3 minutes.
Keep in mind that you can't slurry xanthan gum with water.