Combine milk and water in a large bowl. Dissolve 1 teaspoon sugar and yeast in milk mixture; let stand 5 minutes. Lightly spoon flour into dry measuring cups; level with a knife. Add 2 3/4 cups flour and 1/2 teaspoon salt; stir until dough forms. Turn out onto a floured surface. Knead until smooth and elastic (about 10 minutes); add enough of remaining flour, 1 tablespoon at a time, to prevent dough from sticking to hands (dough will feel tacky).
Place dough in a large bowl coated with cooking spray; turn to coat top. Cover and let rise in a warm place (85°), free from drafts, 1 hour or until doubled in size. (Press two fingers into dough; if indentation remains, dough has risen enough.) Punch dough down; cover and let rise in a warm place (85°) 30 minutes or until doubled in size.
Preheat oven to 475°.
To prepare filling, combine 1/3 cup vinegar and next 6 ingredients (1/3 cup vinegar through garlic) in a zip-top plastic bag. Add pork; seal and chill 30 minutes, turning bag to coat. Strain mixture through a sieve over a bowl; reserve marinade. Place pork in a shallow roasting pan coated with cooking spray. Bake at 475° for 25 minutes or until done. Let stand 5 minutes; finely chop. Pour reserved marinade into a saucepan; bring to a boil. Cook until reduced to 1/4 cup (about 8 minutes). Combine pork and reduced marinade.
Divide dough into 24 portions. Working with 1 portion at a time (cover remaining dough to prevent drying), roll into a 3 1/2-inch circle on a floured surface. Spoon about 1 tablespoon filling onto center of circle. Moisten edges of dough with water; gather edges together to encase filling, pinching edges to seal. Gently twist sealed edges to form a pouch shape.
Line each tier of a 2-tiered (10-inch) bamboo steamer with 3 lettuce leaves. Arrange 6 dumplings, 1 inch apart, in each steamer basket. Stack tiers; cover with steamer lid. Add water to a large skillet to a depth of 1 inch; bring to a boil. Place steamer in pan, and steam dumplings 10 minutes or until dough is translucent (dumplings will be puffy and spongy). Remove the dumplings from steamer. Repeat procedure with remaining lettuce leaves and dumplings.
To prepare sauce, combine 1/3 cup soy sauce, 3 tablespoons vinegar, and seeds. Serve the dumplings immediately with sauce.