1. Place vinegar, mustard, pepper, and salt into the Vitamix container in the order listed and secure lid.
2. Select Variable 1.
3. Turn machine on and slowly increase to Variable 5. Remove lid plug and drizzle olive oil in a slow stream through the lid plug opening.
4. Replace lid plug and slowly increase speed to Variable 10, then High. Blend for an additional 10 seconds.
Refrigerate in airtight container.