At its base vinaigrette is a simple ratio of 1 part acid to 3 parts oil. The fat (oil), and acid (lemon or vinegar), emulsify to create a salad dressing which gets flavors from the addition of salt and pepper as well as shallots and Dijon mustard, both of which also help it to emulsify.
Whisk to combine until the vinaigrette has emulsified.
Season to taste with salt and pepper.
Use or store the vinaigrette in a container with a lid. Store, refrigerated, for 3 to 5 days.
Video
Notes
If serving over salad, test the flavor of your vinaigrette by dipping a leaf of lettuce (or whatever green you are using) in it. You will then know if the flavor complements the salad.