1) combine hoi sin, oyster, and soy sauce with sesame oil.
Chop pork and onions finely.
2) heat vegetable oil in wok or fry pan over high heat. Stir Fry ginger and garlic in the oil one minute. Stir in hoi sin mixture. Cook and stir two minutes. Combine 1/2 cups of water and cornstarch. Blend into hou sin mixture. Cook and stir until liquid boils. Reduced heat to medium and simmer 2 minutes. Stir in pork and onions. Remove from heat. Let cool completely.
3) combine flour, baking powder and salt in large bowl. Cut or cube in shortening until mixture resembles bread crumbles. Combine remaining 3/4 cup of the water and the vinegar. Mix water/vinegar into flour until dough sticks together. Shape dough into ball. knead on lightly floured six or eight times. Cover with plastic wrap and let stand for 20 minutes. Uncover and knead 4 or 5 times. Divide dough into 12 equal portions. Shape each portion into a smooth ball.
4) Roll each ball of dough on lightly floured surface into a circular 5 to 6 inch diameter. Brush around the edges lightly with water. Spoon a heaping tablespoon of pork mixture onto center of each circle. Carefully pinch edges together to seal dough around filling. Bring the two ends of dough over the seam and pinch together.
5) cut wax paper into 12 (5 inch) squares. Brush the up side with oil. Please a bun, seam side down, on each square.
6) Play some buns with paper in single layers on steamer rack over boiling water. Cover and steam buns until done about 20 minutes.