In blender, combine egg, 1 teaspoon oil, and salt. Blend at lowest speed.
With the blender still running, add remaining olive oil in a slow, steady stream until the oil is incorporated. If you prefer a thicker mayonnaise, blend for an additional 10 seconds at a higher speed.
Transfer mixture to a bowl, and stir in the vinegar. Serve immediately, or refrigerate mayonnaise, covered, for up to 2 days.