Make sure the oil is at the same temperature as the milk. You can use them cold, but I found room-temperature milk and oil to be the easiest to work with.
If you're using an immersion blender, combine all the ingredients in the blender beaker or jar, place the immersion blender in, so that way it sits firmly on the bottom of the cup, and pulse until the mayonnaise emulsifies.
Once most of the vegan mayonnaise has been emulsified, you can move the blender up and down to incorporate any oil that is sitting on the top.
I usually use an immersion blender, but I've also tried to make it using a regular blender and it works as well, although I think an immersion blender is the best choice.
If you're using a high-speed blender, add all the ingredients into the blender, except the oil, and blend for about 5 seconds.
Add the oil gradually while the blender is going at a slow speed until it thickens, then you can turn it gradually from low to high and let it go until well mixed.
Whether you're using an immersion or a high-speed blender, try the mayonnaise and add more salt if needed.
If it's too thick, add more milk and if it's too watery, add more oil. Pulse again until it has a perfect consistency.
Use immediately or transfer to an airtight container in the refrigerator until cold.