Gather all ingredients: soy milk, salt, mustard (optional), vegetable oil, and vinegar.

Place soy milk, salt, and mustard (if using) in the blender jar.

Measure out the vegetable oil and set it aside.

Turn the blender on medium-high speed and blend the soy milk mixture for 20-30 seconds.

While the blender is still running, slowly pour the oil into the jar.

After about ten seconds, the mixture should begin to thicken.

Continue adding oil until you reach your desired consistency, varying from watery to very thick.

Add the vinegar and continue blending until you're satisfied with the result.

Taste the mayonnaise and adjust seasoning if necessary.

Blend again for a few more seconds if you made any corrections.

Store the mayonnaise in an airtight container in the refrigerator.