Pour the soy milk into a small bowl or measuring cup and stir in the lemon juice. Let it sit for 5 minutes. The mixture will look slightly curdled, and that's exactly what you want, since it has turned the milk into a tangy vegan buttermilk.

In a medium bowl, combine the vegan mayonnaise with the thickened soy milk mixture and whisk until completely smooth with no streaks left.

Stir in the white wine vinegar, garlic powder, onion powder, salt, and black pepper, then whisk again until everything is evenly combined.

Fold in the chopped dill, chives, and parsley. Taste it, then adjust with more salt or another squeeze of lemon juice if it needs it.

Cover the bowl and refrigerate for at least 30 minutes so the flavors come together. The dressing will thicken slightly as it chills, which is normal.