A PA28 FLYING INTO SAN AREA; IS GIVEN A COMMAND TO CHANGE HIS VECTOR HDG AND IS SLOW TO RESPOND. ANOTHER ACFT PASSES CLOSE BY WHICH CAUSES AN NMAC.

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FLYING CHEROKEE 235 ON IFR FLT IN VMC; IN CONTACT WITH SOCAL APCH; WAS PROCEEDING DIRECT MZB AT 5000 FT (AT APPROX LOCATION NOTED ABOVE). WAS ADVISED BY ATC OF 'VFR TFC; 12 O'CLOCK; 2 MI; OPPOSITE DIRECTION; ALT INDICATES 4800 FT; RECOMMEND YOU TURN R TO HDG 130 DEGS.' I REPLIED; 'NEGATIVE CONTACT; VERIFY YOU WANT ME TO TURN L TO 130 DEGS.' (CURRENT HDG WAS APPROX 145 DEGS.) ATC THEN REPLIED; 'TURN R OR L IMMEDIATELY.' I HAD ALREADY INITIATED A L TURN TO 130 DEGS; THEN QUICKLY SAW THE C172 APPROX 100 FT BELOW AND 200 FT TO L PASS IN THE OPPOSITE DIRECTION. I AM CONCERNED THAT THE TURN ADVISED TO 130 DEGS ACTUALLY PUT ME MUCH CLOSER TO THE VFR TFC THAN IF I HAD BEEN ADVISED TO MAINTAIN PRESENT HDG OF 145 DEGS OR A R TURN TO 180 DEGS. WAS THE C172 IN CONTACT WITH SOCAL APCH EITHER RECEIVING VFR ADVISORIES OR WAS THE ACFT AN INTRUDER IN CLASS B AIRSPACE? IF THE C172 WAS VFR (WHETHER OR NOT IN CONTACT WITH SOCAL) MAINTAINING VFR CARDINAL ALT FOR THAT COURSE (4500 FT OR 6500 FT) RATHER THAN 4800-4900 FT WOULD HAVE ALLOWED ADEQUATE VERT SEPARATION; MAKING THE WHOLE INCIDENT'S OCCURRENCE UNLIKELY.