NMAC BTWN A BEECH V35B LEVEL AT 4000 FT MSL; UNDER IFR IN AND OUT OF CLOUDS IN CLASS B AIRSPACE; AND A SINGLE ENG LOW WING ACFT AHEAD CLBING IN THE SAME DIRECTION. RPTR TOOK EVASIVE ACTION TO AVOID THE OTHER ACFT.

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I WAS BEING RADAR VECTORED BY SOCAL APCH TO LAX VOR WHILE ON AN IFR TWR ENRTE FLT PLAN FROM SNA TO VNY ARPT FLYING AT 4000 FT. THERE WAS A BROKEN LAYER OF CLOUDS AT OUR ALT AND WE WERE PUNCHING IN AND OUT OF THEM. SHORTLY AFTER ENTERING A ZONE OF CLR SKY; THE SOCAL CTLR ADVISED US OF TFC AT 12 O'CLOCK. I LOOKED UP FROM THE FOGGLES THAT I WAS WEARING TO SEE A YELLOW LOW WING LIGHT ACFT ABOUT 300 YARDS AHEAD OF ME AND HOLDING THE SAME HDG AS I WAS. THE BEECH V35B THAT I WAS FLYING WAS INDICATING 165 KTS. THE TFC MUST HAVE BEEN CLBING; AND THEREFORE FLYING AT A CONSIDERABLY SLOWER SPD THAN I; BECAUSE MY RATE OF CLOSURE ON IT WAS EXTREMELY FAST. IT TOOK ME 1-2 SECONDS TO REALIZE THAT; IF I DIDN'T IMMEDIATELY ALTER MY COURSE; I WOULD HAVE CHEWED INTO THE OTHER ACFT'S TAIL. I IMMEDIATELY PULLED UP AND TURNED TO THE L TO AVOID THE AIRPLANE. AS I PASSED ABOVE AND TO THE L OF IT; I KEYED THE MIKE AND SAID TO SOCAL; 'BOY; THAT WAS REALLY CLOSE!' THE CTLR APOLOGIZED AND SAID THAT HE HAD NOT SEEN THE OTHER ACFT. I UNDERSTOOD THIS TO MEAN THAT HE HAD NOT SEEN THE OTHER ACFT UNTIL JUST BEFORE ADVISING US OF IT. MY SAFETY PLT NEVER SAW THE OTHER ACFT EITHER. THE FLT CONTINUED TO VNY WITHOUT FURTHER INCIDENT. THE PROB (IE; THE NEAR MISS) WAS CAUSED PRIMARILY BY THE FAILURE OF THE SOCAL CTLR TO SEE THE TFC SOON ENOUGH TO PROVIDE ME WITH ADEQUATE WARNING OF ITS PRESENCE. MY SAFETY PLT WAS PREVENTED FROM SEEING THE TFC BECAUSE HER FORWARD VIEW OF IT WAS BLOCKED BY OUR OWN ACFT'S STRUCTURE AS THE TFC ASCENDED TO A POS IN FRONT OF US. ADDING TO THE PROB OF SEEING THE EMERGING TFC CONFLICT WAS THE BROKEN CLOUD LAYER THROUGH WHICH WE HAD JUST FLOWN; THE WX CONDITIONS PROVIDED FOR A DANGEROUS MIX OF VFR AND IFR TFC IN CLOSE PROX. IN WX CONDITIONS SUCH AS THAT IN WHICH WE WERE FLYING; WHICH REALLY CONSTITUTED A JUXTAPOSITIONING OF IMC AND VMC CONDITIONS; VIGILANCE BY THE FLC IS CLRLY NOT ALWAYS SUFFICIENT TO PROVIDE FOR TFC SEPARATION THAT WAS NOT SUPPLIED BY SOCAL AT THAT TIME. IN THE FUTURE; SOCAL COULD PREVENT THIS TYPE OF EVENT FROM OCCURRING BY MAINTAINING A HIGHER THAN USUAL LEVEL OF VIGILANCE TO SEPARATE IFR AND VFR ACFT WHEN THE CLOUD CONDITIONS IN THE AREA ARE LIKELY TO PRESENT BARRIERS TO VISUAL SEE AND AVOID PROCS.