In a jar with a tight-fitting lid, combine the vinegar, shallot, Dijon, salt and pepper. Let stand for 5 minutes, then add the oil, tighten the lid and shake vigorously. Taste vinaigrette and adjust seasoning as needed.
When using vinaigrette to dress salad greens, put your clean greens in a large bowl and season them with salt and pepper. Add less vinaigrette than you need and toss your salad with very clean hands. There should be just enough dressing to coat the greens with none left at the bottom of the bowl.
Extra vinaigrette will keep, refrigerated, up to a week. Bring it to room temperature before using.