Toss the cauliflower and leeks with 1/2 cup oil and 1 1/4 teaspoons each salt and pepper in a very large bowl (you may need to do this in 2 batches).
Divide the vegetables evenly between the prepared baking sheets. Roast, switching the pans from top to bottom and front to back halfway through, until soft and browned on the bottom, 25 to 30 minutes.
Meanwhile, place parsley, chives, salt, and pepper in a blender; pulse several times to chop. With the motor running, slowly add the remaining 1 cup oil and process until smooth. Transfer to a bowl.
Transfer the roasted vegetables to a large pot and add the broth. Bring to a boil over high heat. Reduce heat, cover, and simmer for 10 minutes. Puree the soup in batches in a blender. Stir in vinegar and chia milk. Serve with some of the herb sauce on top.