NMAC BTWN AN SMA IN CRUISE AT 1500 FT MSL AND A CESSNA 402 ON VECTOR TO THE DOWNWIND LEG OF THE APCHING DEST ARPT RWY 24 ILS. THE RPTR TOOK EVASIVE ACTION; BUT NONE TAKEN BY THE C402.

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I HAD DEPARTED A SMALL UNCTLED FIELD (5B6) ABOUT 15 MI NE OF OUR DEST; MARTHA'S VINEYARD (MVY). WE WERE OPERATING VFR IN MARGINAL VMC CONDITIONS OF 25000 FT BROKEN AND 3 MI VISIBILITY. FLT TIME TO THE VINEYARD WAS INSUFFICIENT TO OBTAIN VFR ADVISORIES; SO AN IFR CLRNC OBTAINED ON THE GND WAS THE ONLY OPTION FOR RADAR SVC. WE ELECTED TO PROCEED VFR. SHORTLY AFTER DEP AT 1500 FT; I OBSERVED A CESSNA 402C 45 DEGS OFF MY R NOSE AT MY ALT; AND FELT WE WERE ON A COLLISION COURSE. I ENTERED A 45 DEG BANK DSNDING TURN TO THE R; AVOIDING THE OTHER ACFT BY ABOUT 500 FT HORIZLY. THE OTHER ACFT DID NOT APPEAR TO DEVIATE. I BELIEVE THE OTHER PLT DID NOT SEE MY ACFT UNTIL WE WERE PASSING ABEAM IN OPPOSITE DIRECTIONS. THE C402 APPEARED TO BE ON A VECTOR FOR A DOWNWIND LEG TO THE ILS RWY 24 AT THE VINEYARD. WE WERE ABOUT TO CONTACT THE VINEYARD TWR PRIOR TO ENTERING THE CLASS D AIRSPACE WHEN THE CONFLICT APPEARED. THE SEE-AND-AVOID CONCEPT WORKED WELL ENOUGH TO KEEP THE 2 AIRPLANES APART; BUT WITH LITTLE MARGIN; CONSIDERING ONLY ONE OF US APPEARED TO SEE THE OTHER IN TIME TO DEVIATE. WE WOULD HAVE APPEARED IN THE OTHER PLANE'S L WINDSHIELD WHILE HE WAS MORE DIFFICULT FOR ME TO SEE OFF MY R SIDE FROM MY L SEAT POS. THIS INCIDENT CLRLY ILLUSTRATES THE LIMITATIONS OF VISUAL SEPARATION IN LOW WX.