In its simplest, most basic form, a vinaigrette is made of 2 or 3 ingredients - vinegar and oil, of course, and maybe salt and pepper. But you can create an almost infinite variety of vinaigrettes by following a few basic principles and adding or changing the ingredients.
Whisk vinegar, mustard, sugar, salt and pepper together in a non-reactive bowl. Slowly pour in the oil, whisking continually until completely incorporated. Taste and add more vinegar or salt & pepper, if desired.
Experiment with a variety of vinegars (sherry vinegar, red wine vinegar, rice vinegar etc) or other sour, acidic liquids, like lemon, lime, yuzu or other citrus juices, verjus (unfermented tart grape juice) or other tart fruit juices (cranberry, sour cherry or elderberry).
Likewise, try using different oils in your vinaigrette. Nut oils, like walnut or hazelnut oil, have a pleasant, toasty flavor. Avocado oil, grapeseed oil, tea seed oil all make a unique contribution to the flavor of a vinaigrette.
Mustard is optional, but it adds flavor and helps emulsify the oil and vinegar. If your vinaigrette separates, just whisk or shake it well before using.
Enhance the flavor of your vinaigrette with chopped or roasted garlic or shallots. Add a little honey or fruit juice for a hint of natural sweetness. Herbs, like basil, parsley, thyme and rosemary, add dimension and flavor.