AN ACR MLG FLC HAVE AN NMAC WITH A CESSNA ACFT WHILE ON A VECTOR FOR APCH TO LAX. THE FLC REACTS PROPERLY TO A TCASII 'RA' THEN ACQUIRES THE CESSNA VISUALLY. IT HAD CLBED TO THE ACR'S ALT.

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ON DOWNWIND RADAR VECTOR FOR LAX RWY 24R; ATC HAD US AT 2500 FT MSL AND 180 KTS. WE WERE FOLLOWING A FOREIGN ACR'S DC9. WE WERE TURNED TO A HDG APPROX 160 DEGS AND TOLD OF VFR TFC IN TURN AT 2100 FT MSL. I SAW TFC ON TCASII AT 2400 FT MSL AND SAW IMMEDIATE POTENTIAL FOR CONFLICT. I TOLD FO TO GET READY FOR A TCASII RA AS I DISCONNECTED THE AUTOPLT. AT THAT EXACT MOMENT TCASII WENT 'TFC; TFC' FOLLOWED BY 'DSND!' I FOLLOWED STEERING THEN PICKED UP RED AND WHITE CESSNA AT 1 O'CLOCK. INCREASED DOWNWIND MANEUVER AS THE CESSNA PASSED ABOVE AND DOWN THE R SIDE WITHIN 300 FT HORIZ AND VERT. ALT BOTTOMED OUT AT 1900 FT MSL. IF WE HAD NOT FOLLOWED TCASII AND VISUALLY PICKED UP THIS ACFT; IT WOULD HAVE BEEN A MIDAIR. THE GOOD NEWS IS I SAW THIS SCENARIO DEVELOPING BEFORE IT WAS BEYOND OUR CTL. ATC; I THINK; WAS NOT DOING A VERY GOOD JOB OF KEEPING US INFORMED OF THIS 'POP UP' TFC; AND I FEEL THE TURN INTO THIS KNOWN TFC WAS NOT WELL THOUGHT OUT. ALSO; BEING AT 2500 FT MSL AT 17 MI FROM LAX SEEMED A BIT LOW. I AM NOT SURE IF THIS WAS BELOW CLASS B AIRSPACE.