MLG ON VECTOR FOR VISUAL APCH GIVEN SEVERAL TRAFFIC CALLS ON SMT AND STILL HAD NMAC WITH SMT.

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WE WERE BEING VECTORED FOR A VISUAL APCH TO SNA ON A 20 DEG HEADING; LEVEL AT 5000'. CTLR ISSUED A TFC ALERT TO US AT 12 O'CLOCK AND EITHER 5 OR 8 MILES CLIMBING OUT OF 4200'. WE BEGAN LOOKING FOR TFC BUT COULD NOT SEE IT. A COUPLE OF MINUTES PASSED AND APCH ISSUED A FOLLOWUP ALERT - TFC AT 12 O'CLOCK AND 3 MILES CLIMBING OUT OF 4600'. CTLR ISSUED US A 20 DEG TURN TO THE RIGHT FOR VECTORS 'BEHIND' TFC. HE AGAIN ISSUED AN ALERT TO US WITH TFC AT 12 O'CLOCK AND LESS THAN A MILE. TFC WAS NOT IN SIGHT AT THIS POINT. THE CAPT AND I WERE BOTH LOOKING FOR TFC AT 12 O'CLOCK WHEN THE CAPT SPOTTED ACFT AT APPROX 1-2 O'CLOCK. I LEANED AA FORWARD AT THE SAME TIME THE SPOAT THED TFC AND CAUGHT A GLIMPSE OF A LIGHT TWIN (SMT Y) WITH A 'T' TAIL. HE WAS IN A WINGS LEVEL CLIMB AT APPROX 4800' ONE O'CLOCK AND 1 TO 2000' HORIZONTALLY FROM US. THE CAPT CALLED FOR A CLIMB - I WAS ALREADY ADDING POWER AND PULLED OUR ACFT INTO A CLIMBING LEFT TURN. AS I DID THIS THE OTHER ACFT ALSO BEGAN A LEFT BANK AND PASSED UNDER US BY ABOUT 200'. WE REACHED AN ALT OF 5500' BEFORE RETURNING TO OUR ASSIGNED ALT OF 5000'. IN ORDER TO MAKE ALL OF THE LA BASIN SAFE ALL ACFT MUST HAVE ALT ENCODING X-PONDERS (IT SAVED OUR LIVES; THIS TIME). I ALSO BELIEVE THAT POLITICS SHOULD BE REMOVED FROM THE SAFETY ISSUE AND THE TCA EXPANDED TO INCLUDE LGB; BUR; ONT AND SNA ARPTS.