Place all the ingredients in a blender except for the oil. Blend for about 10 seconds or until everything is combined.
Then turn the blender to low and gradually add the oil (while the blender is still going on low). The easiest way to do this is to remove the lid plug (the little plastic plug in the middle of the lid) and pour the oil through it.
When you're done adding all of the oil, put the lid plug back on and turn the blender to high until the vegan mayo is thick enough for your liking.
Notes
Please don't replace the soy milk with any other plant-based milk. The recipe does not work with almond, oat, or rice milk. The mayo will be less thick and creamy.
Also make sure to use unsweetened soy milk. You can taste the difference, even if the soy milk is only lightly sweetened.
If the vegan mayonnaise is too thick, just add a bit more of the soy milk.
If you want, you could also substitute the canola oil with sunflower oil or avocado oil. I would not recommend olive oil as it has a very strong flavor.
Are you a fan of aioli? Then add a clove of garlic before blending the ingredients for the vegan mayonnaise in your blender.