NMAC BTWN A B737-700 FLC AND AN UNKNOWN 'VMC' ACFT 2 NM E OF SLI FOR A VECTORED APCH TO SNA; CA.

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THE NEAR MIDAIR INCIDENT OCCURRED AT APROX XA55 (PST) AT 3000 FT MSL; 2 NM E OF SLI VOR ON A VECTORED HEADING OF 050 DEGS. COAST APCH WAS THE CTLING FACILITY AND WE WERE EXPECTING THE ILS RWY 19R AT SNA. ATC WARNED US OF TFC 12 O'CLOCK POS AT OUR ALT AND THAT WE COULD EXPECT A POSSIBLE TCASII ALERT. WITH TFC APPROX 4 NM AHEAD ATC INSTRUCTED THE TFC TO TURN R IMMEDIATELY. THE TFC WAS SLOW TO RESPOND AND WHEN IT DID IT TURNED L AND DSNDED. WE WERE THEN TOLD TO DSND IMMEDIATELY WITHOUT ANY INSTRUCTION AS TO WHAT ALT OR HEADING. THE FO AND I BOTH WERE READY TO RESPOND TO A TCASII COMMAND WHEN THE RA COMMAND WAS INITIATED BY THE ACFT'S TCASII SYS. THE RA WAS FOR AN IMMEDIATE DSCNT STRAIGHT AHEAD AND THE AURAL WARNING 'DSND; DSND' WAS RECEIVED. THE FO (PF) IMMEDIATELY RESPONDED BY SMOOTHLY FOLLOWING THE TCASII RA DSND COMMAND AND ATC WAS ADVISED OF THE DSCNT. AT APPROX 2600 FT MSL THE TARGET SYMBOL MERGED WITH OUR ACFT SYMBOL WITH A +200 FT DEV. AT 2500 FT MSL; REMOVAL OF THE RA DSND COMMAND AND AURAL 'CLR OF CONFLICT' WAS RECEIVED. AT THAT TIME ATC WAS ADVISED THAT WE WERE RETURNING TO OUR PREVIOUSLY ASSIGNED ALT AND THE ACFT WAS SMOOTHLY RETURNED TO 3000 FT. NO GPWS WARNINGS WERE RECEIVED AT ANY TIME. WHEN CLR OF THE TFC AND STABILIZED TO CONTINUE VECTORS FOR THE APCH; ATC ASKED US IF WE THOUGHT THE VFR TFC WAS IN VMC CONDITIONS. WE ADVISED ATC THAT WE WERE IMC THROUGHOUT THE INCIDENT AND THAT THE TFC WAS AT OUR ALT. VECTORS WERE COMPLETED AND WE WERE CLRED FOR THE APCH. VMC CONDITIONS WERE OBTAINED AT APPROX 2000 FT MSL AND A NORMAL LNDG WAS MADE WITHOUT INCIDENT. AFTER COMPLETING THE FLT THERE WERE NO INDICATIONS FROM THE CABIN CREW OR PAX THAT A NEAR MIDAIR INCIDENT WAS EXPERIENCED. DURING THE ENTIRE INCIDENT SOP'S WERE ADHERED TO AND GOOD CRM PROCS WERE EXPERIENCED. THE FO AND I THOROUGHLY DEBRIEFED THE INCIDENT AFTER THE FLT AND CONCLUDED THAT WE COULD CONTINUE THE OP TO PHX AS PLANNED. NO FURTHER COMS WITH COAST APCH WAS MADE.