Set a medium bowl on a kitchen towel shaped like an "O" (this helps keep the bowl steady while whisking). Whisk together the vinegar, mustard, garlic, 1 1/4 teaspoons salt and a few grinds of pepper in the bowl. Slowly drizzle in the oil while whisking continuously until the mixture is combined and blended well. Use right away, or refrigerate in a sealed container for up to 3 days.