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