THE INCIDENT AIRPLANE ENCOUNTERED VORTEX TURBULENCE WHILE FOLLOWING RADAR VECTORS FOR LANDING SEQUENCE. THE AIRPLANE WAS NOT DAMAGED. A FLIGHT ATTENDANT SUSTAINED MINOR INJURIES DURING THE ENCOUNTER. THE AIRPLANE WAS OPERATING IN A TERMINAL CONTROL AREA UNDER POSITIVE RADAR CONTROL. FAA RECORDS AND RADAR TRACK DATA SHOW A LIKE CATEGORY AIRPLANE WAS APPROXIMATELY 4 NAUTICAL MILES IN FRONT OF THE INCIDENT AIRPLANE WHEN IT DESCENDED TO AND MAINTAINED THE SAME ALTITUDE AS THE INCIDENT AIRPLANE. THE INCIDENT AIRPLANE WAS PROCEEDING ALONG APPROXIMATELY THE SAME GROUND TRACK AS THE LEAD AIRPLANE AND ENCOUNTERED THE TURBULENCE NEAR THE POINT THE LEAD AIRPLANE LEVELED AT THE SAME ALTITUDE. AT THE TIME OF THE ENCOUNTER THE LATERAL SEPARATION OF THE TWO AIRPLANES WAS 4.6 NAUTICAL MILES. THE MINIMUM ATC SEPARATION IN A RADAR ENVIRONMENT WHEN THE AIRCRAFT ARE WITHIN 40 NAUTICAL MILES OF THE RADAR ANTENNA IS THREE NAUTICAL MILES.