Combine the vinegar and milk together and allow it to sit and "curdle" into homemade buttermilk. Whisk all the ingredients together until smooth. Chill for at least 30 minutes - 1 hour. It will thicken more the longer it chills. Serve on a simple salad or use as a dip for veggies.
Sometimes the onion flavor can get a little more strong on day two. If you don't like that flavor, just reduce or omit the shallots.
Notes
Milk substitutes, like oat milk, will not thicken the same as real milk. It must be chilled from 30 minutes to an hour to thicken properly. If it's still too thin, add a little more mayo or consider stirring in a pinch of arrowroot or cornstarch.
This dressing will keep in the refrigerator up to 1 week.