Place a fine sieve over a bowl. Open a can of chickpeas and drain through the sieve. The liquid that will be saved in the bowl is aquafaba. You should get approx. 7 tbsp of it with one smaller can of chickpeas.
Add 7 tbsp of aquafaba, apple cider vinegar, Dijon mustard, and salt to a liquid measuring cup. Use an immersion blender to mix for approx. 1-2 min. until smooth.
While continuing to blend the mixture, add vegetable oil little by little. The volume of the mayonnaise should increase, the mixture also gets brighter and creamier. Keep blending until you reached the desired consistency. Depending on how much aquafaba you used in the beginning, you might need more or less vegetable oil for your vegan mayonnaise. Season with salt and pepper to taste. Stored in an airtight container or mason jar in the fridge, the vegan mayonnaise will keep well for approx. 1-2 weeks.