This is quite a challenge! I started thinking about the mouth feeling of the sour cream, plus thickening power, and I think you may want to go in the direction of nut or seed butter "creams" especially ground unhulled sesame. I will be posting a page on artichokes in March that has two vegan dipping sauces, one of these might do.
Maybe mixed with rice milk or oat milk, some lemon juice or rice wine vinegar. Or walnut, has a bit of bitterness to it that might tend in the direction of sour cream.
When you get what you like, why not send us a recipe? Keep on cooking- Ellen