B737 ON DSCNT INTO SNA EXPERIENCES TCASII RA WITH VFR TFC.

Narrative

NEAR MIDAIR -- RESPONSE TO AN RA. WHILE BEING RADAR VECTORED TO FINAL APCH TO RWY 19R AT SNA WE RECEIVED AN ADVISORY ABOUT A VFR ACFT AT OUR 11 O'CLOCK POS. WE WERE ESTABLISHED ON A 270 DEG HDG PASSING THROUGH ABOUT 6000 FT AND WITH A CLRNC TO 4000 FT WITH THE OTHER ACFT LEVEL AT 4500 FT. WE WERE SUBSEQUENTLY CLRED TO 3000 FT. SOCAL APCH ADVISED THAT THE OTHER ACFT HAD US IN SIGHT BUT WE WERE UNABLE TO SEE IT. VISIBILITY WAS OBSCURED BY HAZE; I ESTIMATE NO MORE THAN 5-6 MI. FO WAS FLYING AND AT ABOUT 5000 FT; I INSTRUCTED HIM TO STOP THE DSCNT AS IT APPEARED THE ACFT WAS HEADED DIRECTLY FOR US 600 FT BELOW AS DISPLAYED ON THE NAV DISPLAY. IMMEDIATELY THEREAFTER; WE RECEIVED AN RA WHICH RESULTED IN A CLB OF ABOUT 200 FT TO 5200 FT. ABOUT 5 SECONDS BEFORE OUR PATHS CROSSED I SPOTTED THE ACFT WHICH APPEARED TO BE A TWIN BEECH PASS DIRECTLY BELOW THE NOSE OF THE ACFT 700 FT BELOW US. IT IS MY ASSESSMENT THAT HAD WE NOT STOPPED THE DSCNT EARLY; THERE IS A VERY HIGH PROBABLY THAT WE WOULD HAVE COLLIDED AT 4500 FT. I FIND IT TOTALLY UNACCEPTABLE FOR ANY ATC AGENCY TO CLR AN IFR ACFT TO FLY THROUGH THE PATH OF ANOTHER KNOWN ACFT IN POOR VISIBILITY WITHOUT CONFIRMATION THAT BOTH PLTS ARE ABLE TO SEE EACH OTHER AND GUARANTEE SEPARATION. BECAUSE OF THE EVENT WE WERE TOO HIGH TO FLY THE APCH AND HAD TO REQUEST VECTORS FOR ANOTHER APCH. IT SEEMS TO ME THAT THE CTLR WAS OVERLY ANXIOUS TO GET US DOWN SO AS NOT TO DELAY OUR APCH.