On May 5, 1999, at 2220 central daylight time (cdt), a Boeing 737-222, N208AU, operated by Vanguard Airlines as Flight 432, encountered turbulence during descent into the Chicago Midway Airport. One flight attendant was seriously injured. The captain, first officer, 2 other flight attendants, and 43 passengers were not injured. The airplane was not damaged. The 14 CFR Part 121 scheduled passenger flight was operating on an IFR flight plan. The flight departed from Minneapolis, Minnesota, at 2108 cdt. The captain reported they encountered about 30 seconds of moderate turbulence while deviating around weather cells during a descent from 17,000 feet mean sea level (msl). He reported the seatbelt sign was on. The captain reported he informed the flight attendants to clean up the cabin and prepare to sit down approximately 10 minutes prior to the encounter. He continued to report that about 5 minutes prior to the encounter the flight attendants were informed to be seated immediately. Upon reaching an altitude of about 10,000 feet msl the cockpit was informed that two of the flight attendants had not been seated and they were injured during the turbulence encounter. The cockpit crew declared an emergency with air traffic control and the flight continued to the Chicago Midway Airport where it was met by medical personnel. The "A" flight attendant who was in the front of the airplane reported that approximately 30 minutes into the flight the captain notified her that they may encounter turbulence and that the flight attendants should secure their galleys. She stated she informed the "B" and "C" flight attendants of the situation. She reported that they secured the galleys and she made a PA announcement that they would be picking up service items. The "A" flight attendant stated that about 15 minutes later she received another call from the cockpit stating that they should secure the cabin and take their seats. She stated she made the final prelanding PA announcement and they secured the cabin for landing. She stated that she was in the forward galley and flight attendants "B" and "C" were in the aft galley when they encountered the turbulence at which time she immediately got into her jumpseat. She stated that after about one minute the turbulence subsided and she made her way to the back of the airplane where she found the "C" flight attendant laying in the aisle and the "B" flight attendant sitting in passenger seat 21C. The "C" flight attendant told her that she had hurt her hip and she wasn't sure she could walk. The "A" flight attendant helped her off the floor and into seat 21D. She then noticed that the "B" flight attendant was bleeding from a cut on her knee. She stated that she then went to the cockpit to inform the flight crew of what had happened. The "B" and "C" flight attendants stated they were informed that turbulence was expected. About five minutes later, as they were securing the aft galley, they were informed to take their seats. They reported the turbulence was encountered as they were going to get in their jumpseats. The "B" flight attendant reported that she grabbed the seatback in front of her and tried to get into a seat in row 21. She stated the airplane dropped and she was thrown to the ceiling where she hit her head. She then came down and landed on the back of seat 20C. The "C" flight attendant was coming out of the aft galley to assist her when the airplane dropped again. The "C" flight attendant was thrown to the ceiling and came back down into the aisle where she landed on her right side, hitting her back against a seat. They stated the "C" flight attendant layed on the floor for about 10 minutes until the "A" flight attendant came from the front of the airplane and helped her into a seat. The "B" flight attendant suffered a cut on her knee and bruises. The "C" flight attendant suffered a fractured pelvis.