In a small bowl, whisk together the vinegar, mustard and salt. While still whisking, add the olive oil in a slow drizzle until the dressing has emulsified (thickened). Stir in the shallots and black pepper.
For a Lemon-Basil Vinaigrette:
Substitute lemon juice for the vinegar, add 1/2 teaspoon of fresh lemon zest, and replace the shallot with 1 teaspoon finely chopped fresh basil.
For a Creamy Vinaigrette:
Add 2 teaspoons pasteurized egg (Egg Beaters or similar), omit the mustard and add 2 teaspoons of heavy cream.