Place soy milk, lemon juice, mustard, salt, optional, and pepper into the Vitamix container in the order listed and secure the lid. Select the lowest setting. Start the machine, slowly increase to Variable 5, and blend for 15 seconds. Remove the lid plug and slowly pour oil through the lid plug opening. Secure lid plug. Increase speed to Variable 10 and blend for 5 seconds, using the tamper to press the ingredients into the blades if necessary.